Attempt to index nil with waitforchild on npc here's my code
local Player = game.Players.LocalPlayer
local Leaderstat = Player:WaitForChild("leaderstats")
local Humanoid = script.Parent.Humanoid
function Dead()
Player.leaderstats.Cash.Value = Player.leaderstats.Cash.Value + 100
wait(0.1)
script:remove()
end
Humanoid.Died:connect(Dead)

2 comments
This is not a script asking site. Try making one yourself, and if it doesn't work, post it here and we'll fix it for you.
-Edited also this is a reward script