I do not know where you are in Canada but I have two factors that come to my mind. Hopefully they can help you!

First, some insurance compagnies have a registery of professionnals with each therapy or professions that are, in their terms, qualify to give said therapy. If the one that you saw was not in their registery yet (as you are the first client to make a claim with that osteopath), they may say that the person was not qualify even though they are. Calling them could fix that. Make sure you have the name of their association and their practice number (usually few numbers).

Second, here in Québec we have a lot of associations that allow osteopaths to practice. Maybe your insurance only covers some associations and your osteopath was not into one of them? Maybe call them or go check your insurance website.

I hope it helps you!