Can you post a screenshot of what you are seeing in Matrix?

It’s possible that BA can sell it but BA.com can’t. Usual advice would be to call but given the current hold times because of strike action that isn’t that helpful.

You could also try using the multi-city tool to add each and every sector separately (i.e. all 5 if routing via London). And/or if you can construct it in Google flights and get the price, click through to BA.com from there.