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Are you trying to use the "Test" keys payment details? (Credit Card Number: 4242424242424242 / CVC : 123)
If yes, then the test mode should be enabled.
If you are not using the "Test" keys, and you got that error, it means that the credit card information is invalid. Please try another credit card information.
Also, it'll be helpful if you can provide a screenshot of the exact error message you're getting and when does it appear.