Task '&' is not fully defined

What causes this issue?

No terminating events are defined for the task used.

How to fix this error?

Please define at least one terminating event for the task used, or mark
the method as a "synchronous object method".

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

Enter the task again in the activity to copy the changed settings to the
workflow.

Error message extract from SAP system. Copyright SAP SE.