No shift abbreviation defined for days off

What causes this issue?

You have overwritten a shift with 'blank'. The overwritten shift was an
employee's shift from a substitution or an employee's shift from the
work schedule rule. The system tries to enter a shift for days off for
this day. However, this is not possible since a shift for days off
cannot be found in the current shift group.

System Response

The current shift cannot be overwritten with 'blank'.

How to fix this error?

Overwrite the shift you want to delete with an allowed shift
abbreviation.

Error message extract from SAP system. Copyright SAP SE.