Any of you know how to buy a game on the playstation store, on ps4?, it keeps saying my card is declined(which my card is not) and I wonder?

Many of those online stores have moved over to two-factor authentication for payments. 

So unless you enter a time-limited code sent to another of your personal devices (typically your phone), your payments will be declined.

I suggest that you check whether you signed up for two-factor authentication but forgot. If you did, then authentication codes may end up being sent uselessly to a device or a mobile phone number which you no longer use.

Some online stores now go further and insist that customers use two-factor authentication.

What kind of card is it?  It is a normal credit/debit card drawn from bank funds?  If so, that should work just fine.  Contact the card issuer for the reasons for the decline.

If the card is a pre-paid debit card, no online store will accept them. You can only purchase items from brick-and-mortal stores.

What I usually do when I have a pre-paid card and want to use it for PSN purchases is go to Walmart, CVS, GameStop, etc. and find a PSN card to purchase and pay for it with the pre-paid card.  Then apply that card to your PSN account for the credit and then you can purchase the game.

Sometimes these declines will put a hold on the account like it was partially accepted but not fully accepted.  It could take up to a week for the hold to lift and those funds to be available to use again.

You might have to call your credit card company to allow you to make online purchases. 

Some credit cards won’t allow you to make online purchases from PSN because it’s been hacked before.