Special period & is not allowed (periods: &, special periods: & )

What causes this issue?

The special period is not allowed.

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?

Check your entry. The fiscal year variant for this company code has
fewer periods or the period entered is not a special period.

Error message extract from SAP system. Copyright SAP SE.