I don't remember getting this message with reicast. Does it show when you're launching the game ? Usually, if the ROM/disc image is not correct, you're returned to the game list. Did you try to launch the same game with flycast ?
Thanks for the input everyone. I've got a handful of working games and I'm happy with that for now. I have far more on my Linux tower RetroPie build that is more fit for this kind of thing.
Your install sounds very out of date. But can't advise without more information apart from to update to a newer version - also 16.04 is probably too old now to work. Recommend 18.04 or 20.04
We do not plan to support other formats at this time.
Neither chdman, nor flycast are developed or maintained by RetroPie. There's no 'plan' for them here - you either use them according to their feature set/limitations or not.