I just bought a Mastercard prepaid and the notice

"This credit card has been denied by the bank that issued it. For details on why your card was denied, please contact your credit card issuer's customer service department. Please enter a different card."

This keeps coming up I am not sure what to do about it as I literally just bought it.

And I am sure answers are easy for anyone else.

Did you try contacting your credit card issuer's customer service department? That seems unlikely to be something SWC can help you with.

Sometimes those prepaid cards require you to go to their website to activate them before they can be used.

What do you mean sometimes. Last I checked any Credit card prepaid or not requires you to activate the card before use.