✓ Accepted answer
Main problem: Script not working
Your script doesn't work because you set the value of the score to itself
game.ReplicatedStorage.Score.Score.Value = game.ReplicatedStorage.Score.Score.Value
I guess what you're trying to do is to set the value in the leaderstats to what is in ReplicatedStorage, so:
game.Players.LocalPlayer.leaderstats.fightscore.Value = game.ReplicatedStorage.Score.Score.Value
Secondary problem:
Inserting a 5-second wait() is really inefficient. Instead, you could use :GetPropertyChangedSignal(string property), so:
game.ReplicatedStorage.Score.Score:GetPropertyChangedSignal("Value"):Connect(function()
Your fixed script would be:
game.ReplicatedStorage.Score.Score:GetPropertyChangedSignal("Value"):Connect(function()
game.Players.LocalPlayer.leaderstats.fightscore.Value = game.ReplicatedStorage.Score.Score.Value
end)