Preceding task &1 not yet blocked by task &2 for &3

What causes this issue?

Task &V1&, which precedes task &V2&, has not yet been blocked for &V3&.
Task &V2& cannot be blocked in this situation.

System Response

Informational message

How to fix this error?

To block task &V2&, you first need to block all of its preceding tasks,
including task &V1& for &V3& in particular.

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.