Are there any ways to create a decline when testing so I can verify my handling of non-approvals?
Which error code are you referring to?
I would like to be able to input a card number or an amount that causes an attempted purchase to decline so I can actively test my code for handling non approval responses from the payeezy server (using the REST and tokenization API).
Many payment providers provide either a trigger amount or special card number that will cause particular types of declines or failures so that the program code that handles those situations can be tested.
Please refer https://support.payeezy.com/hc/en-us/articles/204504175-How-to-generate-unsuccessful-transactions-during-testing-