This problem occurs when the credentials (CCLW, KEY) you use in your integration do not correspond to the environment (Demo/Live) you want to process.
Problem: Merchant Mismatch or Merchant not valid
Solution:
If you want to process real transactions from your customers, you must check that the Web Code (CCLW) you are using is from your Live account and that the Sandbox or Demo mode of your account is disabled.
If on the other hand you wish to process test transactions, you must enable the Sandbox or Demo option in your eCommerce configuration and you must use your Demo account credentials.
If the problem persists, please send me a screenshot of the plugin configuration to [email protected]