MarketplaceService:PromptBundlePurchase() does not work for any free character bundles, and instead gives an Error stating that the item is not currently for sale:
Repro:
game:GetService('MarketplaceService'):PromptBundlePurchase(game.Players.LocalPlayer,985)
*Ensure that Third Party Purchases are enabled in the Game Security settings, and that you do not already own the bundle id which you are prompting yourself to purchase.