-7066: Unique index referenced by foreign key not allowed 

Explanation:

The specified index cannot be dropped, because the columns it contains were specified as referenced columns of the referenced table (referenced_table) in a referential CONSTRAINT definition.

User Action:

You can only delete the index after you have deleted the corresponding referential CONSTRAINT definition. You can use a QUERY statement based on the DOMAIN.FOREIGNKEYCOLUMNS system table to display the CONSTRAINT definitions that were generated.