if you meant that the dropdown is empty, it mean that you never created a region in the DB, you need to create them to be able to set the association to your current entity. (or maybe your current user does not have the access to that entity or association )

there is association and database, even i add default value in that entity region. But not working

validate your implementation against the below and it should work:

• Make sure you have entered records in the Regions entity that are  selectable.
• Make sure the Region entity has the appropriate access rules and the rules are applied to the logged in user.
• If any selectable object condition is added, verify them.

Hope it helps