Time constraint allows you to assign only one person.

What causes this issue?

The time constraint set up for relationships between persons and
positions (subtype &V4&) only allows you to assign one position to on
person at a time. The position assignments of a person may not,
therefore, overlap.

How to fix this error?

* If you want to carry out this assignment, you have the following
options: Delete either the other holder relationship for the period
&V1& to &V2& for person &V3& or set the time constraint for
relationships between person and position (subtype &V4&) to 3.
* Otherwise, you can choose not to carry out this assignment.

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.