Why does this script only change the leaderstat with FE turned off?

I see a few things wrong so let's start from scratch

First let's get our variables in

local ReplicatedStorage = game:GetService("ReplicatedStorage")
local FishEvent = ReplicatedStorage.FISHKINDEVENTS:WaitForChild("YellowFish")

Next let's combine the function and where you call it together

FishEvent.OnServerEvent:Connect(function(plr)

end)

Now for that wait(4)

FishEvent.OnServerEvent:Connect(function(plr)
	wait(4)
end)

Now here's the part about the next two variables, you can't get inside the player's playergui from a script when FE is on. so here what i recommend Inside the localscript you are firing the server from add those two values that we need to check

event:FireServer(xpamount,coinsamount)

now lets update our current script with those variables

FishEvent.OnServerEvent:Connect(function(plr,xpamount,coinsamount)
	wait(4)
end)

now lets add our if statement with the thing you want to do

FishEvent.OnServerEvent:Connect(function(plr,xpamount,coinsamount)
	wait(4)
	if plr:findFirstChild("leaderstats") then
		plr.leaderstats.Coins.Value = plr.leaderstats.Coins.Value + game.ReplicatedStorage.PEOPLE[""..plr.Name..""].FISHING.Coins.Value
		plr.leaderstats.XP.Value = plr.leaderstats.XP.Value + game.ReplicatedStorage.PEOPLE[""..plr.Name..""].FISHING.XP.Value
		plr.leaderstats.Fish.Value = plr.leaderstats.Fish.Value + 1
	end
end)