You shouldn't use the C&C:Online launcher to launch Generals or Zero Hour. Just install
GenTool. When you have done that, you just launch the game from its ordinary shortcut. If you are using the Ultimate Collection, you should install this first;
C&C Ultimate Collection Launchers.
There is a fix for your problem with the other three games in
Common Issues & Solutions (under launcher issues). Please could you try that and if you have no success, let me know and I can give you another solution we're testing.