So you have a published rest service. That service has a problem with its export mapping. Since it is an export, not import, mapping, the mapping is in the Response-tab of your rest-service-activity. The rest service uses a microflow Location_Get_ByKey to fabricate the content of the response and the rest-service uses mapping EM_LocationExport to transform the result of your microflow to the required export format, JSON or XML. The result of your microflow is of entity type System.HttpResponse. The mapping EM_LocationExport however expects a different entity type. Hence this error.
So if you make sure that you make the microflow return its result in a format that the exportmapping can handle, this error will be resolved.
For better understanding / more details on what happens you have two options:
I was under the impression that the mapping was used to convert the domain model entity along with its associations into a formatted JSON string. Then that is put into an http response and exported to wherever it it consumed. That is how the Mendix documentation "Publish a REST Service" does it at least.