In order to play Versus Mutations, or basically, versus in general via single player, you must have at least one addon that changes "sb_all_bot_game" and/or "allow_all_bot_survivor_team" to be 1. One of the following can do that for you:

Competitive-BotsDS By Xanaguy: https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1494592583

Competitive-Bots+ By Omixsat: https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=655424673

Improved Bots (Simple) By Ziggy: https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=121071938

If there are any more addons that enables the game to carry on after all players died, feel free to link them down in the replies.