SAP Message UD603 - Entity type already exists (Please insert in a data model)

Procedure
When you insert an entity type, only the following options areprovided:
1. Create a new entity type (with or without assignment to a datamodel)
2. Assign an existing entity type to a data model
The entity type that was entered already exists, but was not placed ona data model (colored-in area) when it was created graphically.
The insertion process is, therefore, canceled.