I was attempting to create a large map game with 28 opponents. I was unable to start the game because of the "No opponent selected please select at least one opponent" validation message. I was adding the addition opponents by dragging the random opponent card into the opponent listing. The UI also behaved a bit strangely because whenever I attempted to click on one of the random opponents to assign them to a team, it would then reset the number of opponents back to 17.
It appears that problem was that I didn't have enough civilisations to take me up the the 28 opponents. The base game has ~18 civilisations. The solution therefore was to copy all the existing civilisations, then I had ~36 civilisations and could start the game with 28 opponents.
Quite what was happening here was rather confusing initially, I think it would be better if the opponent selection screen simply didn't allow the random opponent tile to be dragged into the opponent list when the number of opponents exceed the total number of civilisations. I think the best way to represent this would be to grey out the random opponent tile with a tooltip stating "All civilisations already in game. Create new civilisations to add further opponents".