Investment key & cannot be deleted (Posting to sub-number &)

What causes this issue?

Maintenance of investment support measures is performed at main asset
number level. Investment support &V1& has already been posted in the
sub-number &V2&. Therefore you cannot delete the investment key &V1&.

System Response

You cannot delete investment key &V1&.

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.

Error message extract from SAP system. Copyright SAP SE.