The activity cannot be changed because follow-up activities exist

What causes this issue?

The current activity already has subsequent activities. Therefore, it
is not possible to change the current activity.

System Response

The activity cannot be changed.

How to fix this error?

At this point, you can only cancel the entire activity and then create
it once again.

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.