There still are entries with higher dunning levels

What causes this issue?

You are attempting to delete a dunning level in a dunning procedure in
which higher dunning levels are entered.

System Response

The system cannot allow gaps in the dunning level table and therefore
prevents you from deleting dunning levels from the list.

How to fix this error?

Select the higher dunning levels as well and then repeat the deletion
process.

Error message extract from SAP system. Copyright SAP SE.