I'm not able to purchase games and apps

Try using web browser to add a credit card under Meta Accounts Center, it's used for identification. There is separately, a Meta Horizon Store that uses the same card for buying stuff (I think this is for buying standalone Quest apps) and an Oculus Store account that uses separate payment credentials, where you can also add payment methods (I think this is for buying Link PCVR games). It's confusing having these three separate areas where payment methods can be set up. Maybe that's the problem. Check all three sites and make sure all have a valid payment method and that none of those payment methods have expired.

Also I suggest setting up a STORE PIN in your headset to prevent unauthorized purchases in case someone hacks your accounts. Be careful not to lose your pin number if you do!