It is bound to Type_ID autonumber field from Customer_Type table. Cannot enter data into autonumber field. This field is automatically populated by Access.
Set form RecordSource to Customers table, not query.
Then bind combobox to Type_ID field in Customers.
Set combobox RowSource to: SELECT Type_ID, Type_Name FROM Customer_Type ORDER BY Type_Name;
This is why it is not a good to use exact same field name in multiple tables.