Mostly, you will face this error when you are trying to use a live payment account credentials to make a test payment on your Test Merchant Account. In this case, you must use "CASHU Prepaid Credentials" at the left side panel. There you'll find the username and password. Use them to complete your test transactions on your Test Merchant Account.
Also, you can face this error if you are using a wrong payment account credentials on the live CASHU payment page.