SNC name of the partner system not in ACL system

What causes this issue?

Communication from another system (R/3 System or other system) using
Secure Network Communications (SNC) was encountered. The SNC name (name
for the external security system) of the R/3 partner system is not in
the table SNCSYSACL for the target system or not permitted for the
corresponding communication mode (RFC/CPIC/DIAG).

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

System Response

The target system terminates the connection.

How to fix this error?

The table SNCSYSACL is an Access Control List (ACL) containing a list
of valid partner systems for SNC protected communication.
If communication was terminated by a valid system, add an entry to the
table SNCSYSACL or activate the relevant communication mode.

Error message extract from SAP system. Copyright SAP SE.