You probably tried to import a new input system package for multiple input devices compatibility. These type of errors are due to conflict between old and new input system packages and are probably resolved in latest updates. To resolve this issue, Go to Edit -> Project Settings -> Player ->Under Other Settings under Configuration is the option Active Input Handling. Select Both. Unity will ...
Ah, but new experts will rise up and embrace the new, friendly Stack Overflow that they have always wanted. And maybe rediscover the same things the bitter, hateful old guard found.
You should use new when you wish an object to remain in existence until you delete it. If you do not use new then the object will be destroyed when it goes out of scope.