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