Answer
Rather than troubleshooting the whole set of changes at once, deploy incrementally and verify after each step — deploy with just the new data source added first (before wiring up the relationship), confirm it succeeds and check the result in SQL Server Management Studio (and optionally in Excel/Power BI for actual data), then add the relationship and redeploy, checking again; this isolates which specific change (the data source itself, or the relationship built on top of it) is actually triggering the DDL failure, rather than guessing at the cause from one all-at-once deployment attempt.