i think there's a confusion that seems a bit more widespread than i thought, so i think it was the wording "select a player" being different from "click a player"
when on the player select screen, you have to select a player first, so if you aren't able to proceed to the game it's probably because you didn't select one. just click any of the characters to select, and then you should be good!
i recorded myself doing so to show it works
this is done so that the game can wait for all players to choose who they want to be so that the game knows who can be controlled and which player is which when handling multiplayer, as well as give the freedom to select which character they like more in singleplayer. it is just clear the wording "select a player" wasn't clear enough, so i'll have to fix that after the jam. good to know and keep in mind for the future!