You cannot delete CO area &

What causes this issue?

You tried to delete controlling area &V1&. This is not possible because
master data (cost centers, activity types, cost elements, or orders) or
transaction data already exist in the controlling area.

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?

You can only delete a controlling area if no master data or transaction
data have been defined for it.
You can display the existing master data by using the master data
indexes in reporting.

Error message extract from SAP system. Copyright SAP SE.