This error can occur when using Payflow with the OptimizeMember Paypal Payments Pro integration.
This is usually caused by your Paypal account not having DPRP enabled
You may need to REQUEST Website Payments Pro 3.0 with DPRP- this is the OLDER API and MUST be requested.
Make this request with Paypal when using Payflow if you receive this error and you should be good to go once this change is made.