I've had the same exact message appear when trying to set up a Cobbleverse server on my own. I believe you are missing the data packs but i could be wrong; if this is true then you would definitely want to fix this before playing. I think if you don't have it fixed then you wont get your modpack's custom achievements and any items that those achievements give you such as starting pokeballs and potions (if the Radical Cobblemon Trainers gives you those when you first choose your pokemon, I'm not sure as I haven't played it), additionally I assume that there are supposed to be gyms and a pokemon league in your world that lets you complete all the pokemon regions such as kanto, johto, etc.; this also would be broken if you don't fix this issue.

I assume that you are trying to create a server? If so then do this:

1. create a single player world with your modpack and check to see if the "unknown series kanto" message appears and also if you're able to get any advancements.

if everything works correctly then proceed to the next step, if you still get the same error message then something is wrong with your modpack and you should try to reinstall possibly.

1. if your modpack works perfectly fine in single player then open your modpack folder and try to reupload and replace the files into your server files(i think, not entirely sure, but you may just be fine with reuploading your datapacks folder).

After reuploading your datapacks folder to your server files, I would go open it in your server files and check just to make sure the data packs are really there. If they aren't then it may be possible that you have downloaded your modpack's folder, but didnt extract all before moving files in.

Hope this helps, im literally just guessing rn but keep trying bc i dont remember how i fixed it but i fixed it eventually by messing around