Transactions can fail for several reasons:
1. Incorrect credit card number or expiration date
2. Minimum purchase amount. The minimum amount will vary depending on your chosen payment currency. For example, to do a test transaction using the Australian Dollar currency, the minimum amount will have to be at least 50 cents
3. Strong Customer Authentication (SCA). For more information about SCA, click on the link here
4. Cards do not allow international transactions. Make sure your credit card supports international transactions.