Period &1/&2 already opened for CG/CU &3

What causes this issue?

Period &V1&/&V2& has already been opened for &V3&. It is not necessary
to reopen the period.

System Response

Informational message

How to fix this error?

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.