Why am I not getting approved for an Apple Card

Please, look for help here --> How your Apple Card application is evaluated

As you are learning, little is more important

in being granted credit than an established credit history.

Your age and effective lack of a history are no doubt significant contributing factors counting “against” you.

Not your fault. Just the current state of the system.

I wouldn't call the Apple Card a "starter card". But, be that as it may, the approval process is not especially transparent. Lots of people with good credit, other cards and without excessive debt have been denied.