Call to undefined method Joomla\CMS\Application\AdministratorApplication::isSit

Good morning, I get this error message when updating from Joomla 3 to 4. Can you then write me how to correct this, because I use another page where this problem will arise. I have already installed the jux easy update. Greetings from Austria Rebecca

Good morning, thanks for the correction, but I have the following problem now, the menu doesn't work on desktop, it doesn't link to the selected pages, but it does on mobile devices. Asking for control. I tried Firefox, Internet Explorer and Edge, all the same problem. Greetings from Austria Rebecca

Yes you can see the menu on the desktop but it doesn't work, click on the submenu and you always stay on the home page. The mobile menu works for some reason. Please correct the error and please let me know what the problem was and how you solved the problem. With best wishes Rebecca

i just added off canvas to the menu on the desktop then you can see the canvas menu to the left of the logo works but the desktop menu doesn't