Shifts could not be swapped on &

What causes this issue?

You cannot swap the shifts of employee 'A' and employee 'B' in the
following cases:
If employee 'A' and/or 'B' is absent for the entire day
If employee 'A' and/or'B' is assigned to a foreign requirement
If employee 'A' and/or 'B' does not have a shared temporary assignment
If the shift is not a <DS:DE.DAYPROG>specified shift</>

System Response

The shifts are swapped.
&WHAT_TO_DO& &SYS_ADMIN&

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.