That warning is generic — the key is what the SaaS connector logs show when it fails. Since NERM is an OOTB SaaS connector, grab logs via SailPoint CLI: sail conn logs tail while running Test Connection / aggregation.
Once you see the actual HTTP status:
401/403 → API key/permission issue (similar NERM case fixed by generating a new NERM API key and updating the source).
429 → throttling; reduce paging/page size if configurable and retry.