Do you have flutter packages pub build_runner watch running?
Kill all dart processes and try again or restart the computer.

@sathishbalan is this still an issue?

Might be a dup of #14703

yes, get it; it is work

Kill the terminal using the delete (trash) icon if you are using a Visual Studio Code editor

Without additional information, we are unfortunately not sure how to resolve this issue. We are therefore reluctantly going to close this bug for now. Please don't hesitate to comment on the bug if you have any more information for us; we will reopen it right away!
Thanks for your contribution.
cc @Hixie

Hi!
i'm still having the same issue and @zoechi didn't work for me.

What I noticed is that there is an issue with the 'pub' tool ==> 'build_runner is not found' and that there is an exit code 64 for involved (what ever that means) I looked it up in the documentation, but without any success...

Hopefully, this will be useful for some one to solve the solution!

@D19cannon Please create a new issue with full information, including the output of flutter doctor -v.
Please check first you have a recent Flutter version.

Hi, yes I do the latest (upgraded a billion times).
Any how, I solved it by removing the .bin/lock files (from another issue) or and used the command afterwards.

You can: rm ./flutter/bin/cache/lockfile

This issue also occurs due to internet proxies ,make sure that you have full network access .

you can open flutter/bin/cache/lockfile ,and delete this lockfile ,I solve this problem by this

On OSX:
rm /Applications/flutter/bin/cache/lockfile

On OSX:
rm /Applications/flutter/bin/cache/lockfile

delete 'lockfile',It is work

Thanks ! @CompileYouth

On OSX:
rm /Applications/flutter/bin/cache/lockfile

Thanks. But mention that the lockfile file is depending on the location that flutter SDK. In my OS，I need to delete ~/Documents/SDK/flutter/bin/cache/lockfile.

another way On OSX:
open activity monitor and find process named 'dart', kill it.

This issue also occurs due to internet proxies ,make sure that you have full network access .

And what about if we have a proxy. What can we do to fix it?

In my case, there was an flutter packages get command running in the background. As it was looking for package that did not exist due to incorrect name, changing this package name fixed the issue.

[eamper-app] flutter packages get
Running "flutter packages get" in eamper-app...                 
Because eamper depends on ffirebase_core any which doesn't exist (could not find package ffirebase_core at https://pub.dartlang.org), version solving failed.

pub get failed (69) -- attempting retry 1 in 1 second...

Because eamper depends on ffirebase_core any which doesn't exist (could not find package ffirebase_core at https://pub.dartlang.org), version solving failed.

pub get failed (69) -- attempting retry 2 in 2 seconds...

Because eamper depends on ffirebase_core any which doesn't exist (could not find package ffirebase_core at https://pub.dartlang.org), version solving failed.

pub get failed (69) -- attempting retry 3 in 4 seconds...

Because eamper depends on ffirebase_core any which doesn't exist (could not find package ffirebase_core at https://pub.dartlang.org), version solving failed.

pub get failed (69) -- attempting retry 4 in 8 seconds...
Running "flutter packages get" in eamper-app...                    23,0s
exit code 0
[eamper-app] flutter packages get
Running "flutter packages get" in eamper-app...                     1,4s
exit code 0

Find the folder where flutter is installed, and delete flutter/bin/cache/lockfile

run flutter clean

flutter clean won't run either. If I delete the flutter.bat.lock file, it will run clean, but the "Waiting ..." message still comes up when I restart the idea. Have to reboot my computer to be able to delete lockfile, even when the IDE is shut down. Doesn't fix the problem.

flutter clean, changing networks, deleting lockfile, killing dart processes and restarting computer did not work for me. I'm starting to think maybe this happens on specific devices. Going to try that.

I suspect flutter would get more corporate adoption if it were easier to use in a corporate environment. Might want to figure out a way to remove all the proxy hoops that need to be jumped through to use it.

Restarting the terminal worked for me.

Still having this problem, nothing seems to clear it.

killall -9 dart

delete lock file not worked for me.

You can: rm ./flutter/bin/cache/lockfile

that'is it,thx

I had tried all the solution mentioned above, and nothing works for me and creating flutter application dialog goes forever, but what I did is, end AS task and open it again but this time instead of creating a new project, I just opened the project I just exit from it, and it opens fine,
what causes this problem, I think when I tried to load a picture from image.asset
after creating a new dir asset/images/pic.jpg and add the asset in pubspec.ymal
when getting the package or something, that's when all this happened
Conclusion:
just create a new project, and when Creating flutter project dialog appear close Android studio from Task manager, then open it again but this time open an existing project which is the previous one
hope that works for you

there may be a way to resolve this problem
just install another version of dart
update environment variables
it will be fixed

On windows open task manager and kill the last dart child process in android studio

just delete the flutter/bin/cache/lock file and then do flutter upgrade in ide and it will solve your problem.

just remove lock file and restart my Mac work for me

I solved the problem by opening task manager and killing the 'dart.exe' process(es)

You can: rm ./flutter/bin/cache/lockfile

Work for me,
thank you

@CompileYouth That's wored for me. Thanks

In my case, following command in Terminal or cmd helped (as suggested by Günter):

killall -9 dart

On Windows (as suggested by upupming):

taskkill /F /IM dart.exe

or remove this file

<YOUR FLUTTER FOLDER>/bin/cache/lockfile

This releases the occupied lock and makes you able to run other commands.

On Mac, open Activity Monitor, search dart, then quit all dart process

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.