This problem usually happens when the repack uses a fixed offline profile or blocks online features, preventing you from entering a custom name.

UserName=SKIDROW

(often found in various repacks), you typically need to replace a specific library file that is responsible for handling the Steam API emulation. The "Player One" Name Fix

If your game crashes when you try to change your name or save, it may be a directory issue. Run as Admin : Always run the game and your text editor as an Administrator to ensure files can be saved. Disable Steam Cloud

You cannot change it inside the game because the emulator tells the game that your profile is read-only.