I never heard of bru, but it sounds a bit like tar.

To extract those files, you need to write them to disk. Do you have permission to do that?

Does /mydir exist? If not, you will need to do the equivalent of "mkdir /mydir". That requires write permission to /. Usually only root could do that.

If /mydir exists, can you write to it? You need to create files in it and that means write permission to /mydir.

Or if those files already exist, can you write to them?