Deletion not possible (asset subnumber &1 &2 still exists)

What causes this issue?

You cannot delete the main asset number, since at least one sub-number
(sub-number &V2&) still exists.

System Response

You cannot delete main asset numbers when sub-numbers are still active.

How to fix this error?

You have two options:
Cancel the deletion.
Delete all sub-numbers first, and then delete the main asset number.

Error message extract from SAP system. Copyright SAP SE.