Hi,
I just tried to remove the .build folder and I got this error:
Error Domain=NSCocoaErrorDomain Code=513 "You don’t have permission."
But I'm the administrator of my computer and yes, I tried to remove it "as administrator"
Plus... Cocoa? I'm on Windows... 
Windows 11
Swift 5.9.2
Any idea?
It could be that something is holding on to the directory. You may have it open in explorer, in a terminal, or perhaps an editor?
Thank you Saleem for your reply.
Yes I double checked if any process was blocking access, as any windows user painfully learnt to do 
I use one of the tools in windows very own power toys. However the tool couldn't find anything.
Obviously a process was holding those files though, because after a full reboot, I was able to delete the folder.
This is not the first time I encounter that issue, so I was hoping it was something common and that maybe a simple fix was known.
Not necessarily - it could have been a zombie lock. The process could have terminated without relinquishing a lock, the kernel could have inherited it. The way to get around deletion is to use something like:
MoveFileExW(pwszFile, L"NUL", MOVEFILE_DELAY_UNTIL_REBOOT);
This allows for deleting the file on the next reboot.