You cannot maintain a service date for installation &1

What causes this issue?

No valid operand could be found for installation &v1&.
You can only specify a service date if the installation has a valid
combination of operand and reference value. The operand must have
operand category <LS>REFVALUE (reference value)</> and reference value
type <LS>Container</>. A container must also be allocated to the
reference value.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Enter a valid operand for the existing installation in the installation
facts.
If the operand is not valid until a later date, you must only change
the key date of the service frequency.

Error message extract from SAP system. Copyright SAP SE.