I am having an issue in Unity where every time I launch the editor it asks me to enter in safe mode because there are a lot of errors and when I press ignore it launches and I can see in the console a lot of errors with the UI and I found out that I cant create UI elements directly in the hierarchy. I tried to update from 2020.3.13f1 to 2020.3.14f1 and for the first launch everything was fine, with no errors, and I could create UI elements but then when I restarted to make sure everything was actually fine, all the errors popped up again and I cant create UI. Why does this happen and does anyone have a fix because I can't find it anywhere online.
Try not to enter Safe Mode. Enter the editable project. There go to Assets-Reimport All.
It did it to me too, it's probably a Unity bug, this method fixes it. Practically it reimports the whole project by itself, avoiding that the bug gives you problems (such as those like UnityEngine.UI does not exist or Monobehavior does not exist, etc.)
Obviously you will not lose any assets and everything will be fine as before receiving the errors.