My credit card was declined when checking out
When you’ve decided to check out, the charge is submitted to your bank or credit card provider. They have automated systems that will determine if they should authorize or decline the payment. The systems look at things like your spending habits, your account balance, and if you make international purchases, etc.
Call your credit card support number
This can be solved quickly by calling the number on the back of your credit card and explaining that you’d like the charge to go through.