Key date is not within a valid subscription right period

What causes this issue?

Since the key date does not fall within a valid subscription right
period, you cannot exercise the subscription right.

How to fix this error?

Check the subscription right periods in the class data and the key date
of the exercise or choose another subscription right period, if
required.

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.