Interface could not be determined for proxy &1 (method &2)

What causes this issue?

An interface in the SAP Application Interface Framework could not be
determined for this message.
For the Proxy class/Proxy method combination, no interface has been
assigned.

System Response

In a scenario where the IDoc is to be processed in AIF, the system sets
the message to an error status (Application Error). Because the Proxy
message could not be processed.
In a scenario where the Proxy message processing is only to be monitored
in AIF, there is just this error message written to the application log
for your information. Because the Proxy message might not be in the
monitoring scope by intention.

How to fix this error?

Check if a valid Proxy class/Proxy method combination is set. If not,
change the settings in Customizing for <LS>SAP Application Interface
Framework</> under <LS>Interface Development</> ->
<DS:SIMG./AIF/INTERFACES>Define Interfaces</> and restart the message if
in an error status.
Check which fields should determine the interface. You may then do one
of the following:
Change the Customizing in <DS:SIMG./AIF/INFDETP>Interface Determination
for Proxy Interfaces</> and restart the message.
Cancel the message and send it again from the sending system (if the
data is wrong).

Procedure for System Administrators

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.