Why is my 'LoopKill' command erroring?

Lua's line numbering can sometimes be a bit off.

In this case, your code is much messier than it should be.

Here's the same thing with neater spacing, better variable names (and the if reduced to the one case that always occurs):

if msg:lower():sub(1, 9 + #Prefix) == Prefix.."loopkill " then
	local chk = msg:lower():sub(10 + #Prefix):find(" ") + 9 + #Prefix
	local players = GetPlr(plr, msg:sub(10 + #Prefix, chk-1))
	local num = #msg:sub(chk + 1)
	for _, player in pairs(players) do
		coroutine.wrap(function()
			if player and player.Character and LoopKill then
				local LoopKillPlr = LoopKill:Clone()
				LoopKillPlr.Parent = game.Workspace
				LoopKillPlr.Disabled = false
				LoopKillPlr.Name = "LoopKill-" .. player.Name
				local ConfirmName = Instance.new("StringValue", LoopKillPlr)
				ConfirmName.Value = player
				ConfirmName.Name = "loopKill"
				local Kills = Instance.new("IntValue", LoopKillPlr)
				Kills.Value = num
				Kills.Name = "Kills"
				table.insert(Container, LoopKillPlr)
			end
		end)() --This was line 962
	end
end

I think the problem is probably

ConfirmName.Value = player

Value ought to be a string, but you are giving it a object. Try using

ConfirmName.Value = player.Name

#Asides

Your computation of chk could error when :find returns nil.

There's also no reason at all to use coroutines or even a function here where you are.

Also, in the if, you shouldn't have to check the existence of player.

if msg:lower():sub(1, 9 + #Prefix) == Prefix.."loopkill " then
	local chk = msg:lower():sub(10 + #Prefix):find(" ") + 9 + #Prefix
	local players = GetPlr(plr, msg:sub(10 + #Prefix, chk-1))
	local num = #msg:sub(chk + 1)
	for _, player in pairs(players) do
		if player.Character and LoopKill then
			local LoopKillPlr = LoopKill:Clone()
			LoopKillPlr.Parent = game.Workspace
			LoopKillPlr.Disabled = false
			LoopKillPlr.Name = "LoopKill-" .. player.Name
			local ConfirmName = Instance.new("StringValue", LoopKillPlr)
			ConfirmName.Value = player
			ConfirmName.Name = "loopKill"
			local Kills = Instance.new("IntValue", LoopKillPlr)
			Kills.Value = num
			Kills.Name = "Kills"
			table.insert(Container, LoopKillPlr)
		end
	end
end