On-screen indicator for brightness level or sound level not showing up when changing settings
I would suggest reinstalling macOS. Shutdown your computer, power it back on and immediately press and hold Command + R. When it finishes booting, select your language and select Reinstall macOS. Follow the prompts. Apple support link for more info. This shouldn't overwrite your personal data, but I always recommend having a backup just in case.

It's generally not a good idea to be deleting or changing files in the System or Library folders unless you know exactly what the file does and you are willing to accept the risk of an unstable or broken system.