"Play" mode unable to find the player's Gui?

As with a lot of other posts, this problem is not in test mode but rather in the Roblox "Play" mode. I have a server script that will detect when a player is inside the part "ENTERSHOP" and will make the player's shop GUI visible. When the script detects a player in the Roblox server mode, it is seemingly unable to find the ScreenGui "Gui" as a child of PlayerGui and throws an "Infinite yield possible" error relating to line 10

local part = script.Parent
local debounce = false

part.Touched:connect(function(hit)
    if debounce == false then
        debounce = true
        if hit.Parent:FindFirstChild('Humanoid') then
            local plr = game.Players:GetPlayerFromCharacter(hit.Parent)
            if plr then
                if plr.PlayerGui:WaitForChild("Gui").Shop.Visible ~= true then
                    plr.PlayerGui:WaitForChild("Gui").Shop.Visible = true
                end
            end
        end
        debounce = false
    end
end)

part.TouchEnded:connect(function(hit)
    if debounce == false then
        debounce = true
        if hit.Parent:FindFirstChild('Humanoid') then
            local plr = game.Players:GetPlayerFromCharacter(hit.Parent)
            if plr then
                if plr.PlayerGui:WaitForChild("Gui").Shop.Visible ~= false then
                    plr.PlayerGui:WaitForChild("Gui").Shop.Visible = false
                end
            end
        end
        debounce = false
    end
end)

I know that the scripts are a little sloppy but I plan on cleaning it up after the bugs are fixed.