If you are using the Hosted Payment Pages (HPP) integration and are seeing the error Merchant signature was not valid, this means that the HMAC signature you are sending to us is not corresponding to the HMAC signature we calculated. This indicates that your HMAC signature is not being calculated correctly. 

For all details and steps on how to implement HMAC signature calculation, please refer to our documentation page HMAC signature calculation.

Please note that the Hosted Payment Pages (HPP) integration is being deprecated. Find out more about the time-line and what you need to do here.