In flutter sometimes you might get stuck in the "Waiting for another flutter command to release the startup lock..." issue:

I also got stuck here so let me provide you with a solution that didn't work for me:

1. I tried this one from StackOverflow which was a similar case but it didn't work

   $ killall -9 dart

2. Then I ran these commands:

   $ flutter doctor

   $ flutter

   But both gave me the same error

Then What worked for me was:
Removing and Installing Flutter

I am on Linux so I did this else you can refer to Official Documentation for installation and removing

$ sudo snap remove flutter

$ sudo snap install flutter --classic

Then I tried to run $ flutter doctor command and it worked for me

BOOYAH! It finally worked for me.

And I recommend you reboot your system after this!

Hope you also solved your issue :)

Best Wishes!