Hi I'm new to coreboot and visiting briefly, I'm replying to this error because it looks like a systems error rather than a coreboot error, at first glance.

"permission denied" during any build of anything is usually from accidentally running part of the build under "sudo", and fixed with (sudo chown -R user:user .) replacing "user" with your username, to copy your parentheses syntax.

Another usual fix is to wipe the source tree and clone the repository again.

What operating system are you building on?