Had to look it up, but the purpose of .nomedia is to tell apps not to scan that directory for content. It's basically a suggestion, but say you have a dir with a bunch of cached web assets; it would be useful to drop a .nomedia file in there. So really this seems like a bug in the screenshot tool for being too strict (it isn't supposed to prevent writing, only reading). Like the screenshot tool tries to create the dir when it already exists (but can't be seen), fails, and gives the wrong error message.