In some cases, a program which cannot be quit during normal operation (like the Finder or the Dock) could cause a technical problem with your computer. Such a problem becomes even more severe if automatic login of a user is active, so the erroneous application is becoming active by itself after each startup. To resolve such a problem by using a second user account, the automatic login of a user after startup can be switched off by TinkerTool System for Recovery Mode.
Automatic login can be reenabled later if desired, by selecting System Settings > Users & Groups > Automatically log in as… in macOS.
It is possible to make configuration changes to the operating system that inadvertently disable all administrative user accounts. This is a very critical situation because you may lose access to the system and also can no longer authenticate in order to fix this. The easiest solution to resolve such a situation is to force the operating system to re-run its Setup Assistant, the application that is usually started after you have installed or upgraded the computer for the first time. The macOS Setup Assistant will allow you to recreate the primary administrative user account, without losing or changing any other data.
To force the system to re-run its Setup Assistant the next time the computer is started, perform the following steps:
After that, you can use ttsfrm > Quit ttsfrm and restart the computer via the Apple menu to let the system start with Setup Assistant.