Windows 10 does not let me add tiles to my start menu and I do not know why.
I had a school account on my laptop but I removed it and everything else was fixed but not my start menu settings.
Below are some fixes that might help in your case. I suggest before starting to create at least a System Restore point, so to have an easy way of undoing them, just in case.
Allow full Microsoft telemetry
Later on you could return this to "1 - Basic", if you wish.
Check for corruption
Check for Windows system errors using the following: