After making a test transaction, you may have received the error “Form transaction registration failed.”
The error would appear when the transaction request to process the payment form has a problem with the post’s format or the encryption of it when the request is sent to Opayo’s (formerly SagePay) system.
To resolve this, double-check the Encryption Password that you have placed on your plugin settings:
You can get the “Encryption Password” or the Encryption key from your Opayo’s (formerly SagePay) account dashboard > Administrator > Password Details > Form Integration Encryption Password.
To learn more steps on how to resolve it, you can check out Opayo’s (formerly SagePay) knowledge base article here https://www.sagepay.co.uk/support/error-codes/5080-error-form-transaction-regisation-failed.