Hi,
This could be anything. but 99% it is related to php7.1
getParam() doesn't give much of detail. But there is one thing i know 100%, this error message is not generated by any of the files of MaterialMart, because we don't use getParam() method nowhere within MaterialMart.
If you are getting this error message, only when you get response from the payment method, then you should better check the payment plugin, try updating it. Payment plugins usually have a custom-made response page to display the order details and the "thank you" message. The URL when you get this error message will most likely help you to track down the error. First, you have to find what plugin/component is generating this message, then seek a solution.
Regards