What does it mean when my card is declined for "reserved use"?

the way I like to make sense of the too-complicated use of credit cards is the restaurant model.

After your meal, you give the wait person your card, and they then place a HOLD or Reserve or Pending Sale Amount on the card (presuming you are good for about that amount, possibly grossed up to include an expected tip).

NB> If you are near your overall credit limit, that Reserved Amount can NOT be spent on other purchases.

They return to the table with the slip for you sign. At that point, you may decide to add a tip, have another drink or make some other changes.

At some point in the near future (depending on accounting practices that may be right now, or sometime in the next few days) the final transaction will be processed and that amount will be charged from your account and scheduled to be paid to the restaurant.

If you abandon a purchase in progress, the Reserved Amount may still be pending on your Account for several days. Since that decreases your overall credit limit, you will not be able to schedule new purchases -- because your limit has [technically] been exceeded. Your card will be refused.

Any fixes can come from waiting a few days, or possbly by contacting your credit card issuer.

Apple is not a bank and cannot approve or decline a payment/transaction. Please contact the support team at the bank that issued the card that was declined. Apple Pay transaction are anonymous to Apple and has no information on the reason for a decline, items purchased, card used or other details about the transaction.

Maybe it’s already associated with another account?