Why am I told my import isn't supported?

This used to mean a hard stop, but it doesn't anymore. If the source SDS uses a proprietary blend, or lists ingredients without CAS numbers, ExactSDS still imports it -- while you're reviewing the extracted data, you'll see a reminder that each component needs a CAS number, and for any the supplier withheld (a proprietary blend, or an in-house chemical), you mark that component as having no CAS number right there in the review step, instead of the import being rejected.

It's saved the same way as any other no-CAS ingredient (see “Can I add a substance that doesn't have a CAS number?” in the Substance & Mixture Library section) -- you no longer need to abandon the import and rebuild the SDS manually just because a supplier didn't disclose a CAS number.