The “The transaction cannot be completed due to an invalid merchant configuration” error message is displayed when you are using PayPal and you do not have a PayPal "PRO" account.
The PayPal Pro account is one step higher then their Business accounts. PayPal charges $30 per month for a Pro account. If your account does not incur a monthly service fee of $30, you do not have a PayPal Pro account.
Please note, it will take several days for Pro features to be active following an account upgrade.