You can only enter a special period for the last posting period

What causes this issue?

You are making postings in the FI-SL application component. Based on
your Customizing settings, postings are to be made to ledger &V1&. This
ledger has the fiscal year variant &V2& (for the company code in which
you are posting). Posting period &V3& was determined based on fiscal
year variant &V2& and the posting period you entered, but it is not the
last posting period of the year. Thus, the entry of a special period (
&V4&) is not allowed.

System Response

The posting is not allowed.

How to fix this error?

Change the posting date so that (considering fiscal year variant &V2&)
it lies in the last posting period, or do not enter a special period.
You may not want to post to ledger &V1&. In this case, restrict its use
in the ledger's Customizing.

Procedure for System Administrators

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.