Create a start condition; click the required event linkage

What causes this issue?

In the mode <ZK>Create start condition</>, a list of event linkages is
displayed to which no start conditions have yet been assigned. The list
is compiled in accordance with the search criteria you specified in the
upper screen area. Single-clicking on an entry in the list (= an event
linkage) calls the condition editor, with which you can now enter a
start condition for this event linkage. When you return from the
condition editor, you automatically return to the mode <ZK>Edit/display
</> <ZK>start condition</>, in which you can modify the start condition
and the associated attributes of the start condition.

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.