If you are still having issues after checking your work:

Once you have your 'mods' folder set up (if you don't already) download this Mercedes-AMG GT63 S Coupe [Add-On / OIV | Tuning] 1.2 & install it (to the mods folder, option in '.oiv' install dialogue) using the 'RmodCustoms_AMGGT63S.oiv' in the download.
It will install everything required for the car to spawn automatically & if it spawns (using the name 'rmodgt63') will confirm whether everything is set up correctly & nothing else is stopping add-ons from spawning etc