I would be using PlayerMembershipChanged instead, because .PromptPremiumPurchaseFinished returns nothing. You can tell it returns nothing, because on the documentation, there are no parameters.
Code:
local Players = game:GetService("Players")
Players.PlayerMembershipChanged:Connect(function(player)
if player.MembershipType == Enum.MembershipType.Premium then
print("Premium user")
end
end)