You can only post in new year after closing the previous year.

What causes this issue?

You want to post in a new fiscal year. However, a posting must have
been made in the last period in the previous fiscal year before you can
do this.

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?

Post to the last period in the previous year first.

Error message extract from SAP system. Copyright SAP SE.