Data is incomplete (12/31 is missing)

What causes this issue?

To define a fiscal year variant fully, you must make entries for each
day of the calendar year. This means that it must be possible to
determine the corresponding posting period for each calendar day.
This also includes an entry for the last days of the year. This entry
is missing here.

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?

Specify a posting period for the last days of the year.

Error message extract from SAP system. Copyright SAP SE.