Columns names are case sensitive on postgres
When migrating, did the migration script double-quote the column names when creating tables? That would preserve the case.

You could either fix the migration script and re-import, or rename the columns e.g.

ALTER TABLE my_user RENAME COLUMN "USER_ID" TO user_id;


(note: user was used in the original question but that's a reserved keyword in Postgres so that couldn't have been one of the tables involved)