No unit is defined for 'DAY'

What causes this issue?

No unit is defined for <LS>DAY</> in your system. If you are working
with maintenance plan scheduling, this unit must be set.

System Response

INCLUDE KEINE-FUNKTION OBJECT DOKU ID TX LANGUAGE EN

How to fix this error?

INCLUDE SYSTEMVERWALTER OBJECT DOKU ID TX LANGUAGE EN

Procedure for System Administrators

Define a unit for <LS>DAY</>.
IF &DEVICE& = 'SCREEN'
<DS:TRAN.CUNI>Execute function</>
ENDIF

Error message extract from SAP system. Copyright SAP SE.