If you have disabled the launcher and want to enable it again, you can do so in-game. Start the game, select Settings, go to Game, scroll down to General and set the Show Launcher toggle to On. If you have disabled the launcher and are unable to start the game, you can force the game to use the launcher by adding a command line argument.
On Steam, right click Ratchet & Clank: Rift Apart in your Library, select Properties, go to General. Add -launcher in the field for Launch Options.
On the Epic Games Store, click your profile, select settings and scroll down to Ratchet & Clank: Rift Apart. Tick the box for Additional Command Line Arguments and add -launcher in the field below.