Description
The domain you are trying to delete is being used by a subtype. You cannot delete a domain if it is in use.
To delete a domain, it cannot be assigned to any fields in a table (with or without subtypes) or be a required domain in the utility network.
Solution
Unassign the domain from any datasets that are using it and attempt the operation again.
To learn how to discover datasets that are using a specific domain, see Identify domain usage.