You probably need to call your bank.
When I was younger & got my first bank account, I remember trying to buy computer parts and having the same problem.
A ton of bank have online fraud protection & won't even let you buy certain things over a certain amount.
Like with the bank I have now, If spend a few hundred dollars on something online, I get a phone call within a few minutes asking if it was really me that made the purchase.
Call your bank, explain what you are trying to do & your card should stop declining, assuming that's really the problem. IDK though just guessing.