Plan version &1 in controlling area &2 is locked for fiscal year &3

What causes this issue?

Plan version &V1& is locked for Profit Center Accounting in controlling
area &V2& for fiscal year &V3&.

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. If necessary, unlock the plan version for Profit
Center Accounting. To do so, select the function "Planning ->
Plan versions".
<DS:TRAN.OKEQ>Proceed</>

Error message extract from SAP system. Copyright SAP SE.