Fast Serialization.

To ensure that fast serialization executes the appropriate
interfacecheck, it is essential that you answer these questions in the
configuration:
Do you want to use fast serialization for an existing or a new scenario?
Only if you have an existing scenario, you must also tell the system<(>
,<)>
Whether you have used classic serialization, basXML or enforced basXML
before.
Whether the server or the client of this destination is an S/4HANA
release.
For more detailed information on the different interface checks see note
2284997.
You can only use fast serialization, if the server side also has this
feature. (For release information on fast serialization look at note
2418683.
To optimally speed up your performance with fast serialization, it is
important to maintain the information whether or not you have a slow RFC
connection/WAN-scenario. (selection field above)
Select ?Configuration Check? at the top of the page in order to validate
the information you have entered for items 2 and 3.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.