"FireClient: player argument must be a Player object?" Even though it is??

you shouldn't have a function inside a loop, since it will continuously create a new function when it really isn't necessary. you also need to have the Touched event check if the part touching is an actual player so that it can fire.

fix:

local Projectile = script.Parent
local Heartbeat = game:GetService("RunService").Heartbeat

while Heartbeat:wait() do
    Projectile.CFrame = CFrame.new(Projectile.Position, Projectile.Position + Projectile.Velocity)
end

Projectile.Touched:Connect(function(hit)
    if hit and hit.Parent and hit.Parent:IsA('Model') and hit.Parent:FindFirstChild('Humanoid') then
        local Player = game:GetService('Players'):GetPlayerFromCharacter(hit.Parent)
        local Model = game:GetService('ReplicatedStorage').Morphs.SweetCheeks
        local Event = game:GetService('ReplicatedStorage').Events.Morph
        Event:FireClient(Player, Model)
    end
end)

Morph Script

local event = game.ReplicatedStorage.Events.Morph

event.OnServerEvent:Connect(function(player, model)
    print("--------")
    print("Player: "..player)
    print("--------")
    print("Morph: "..model)
    print("--------")
    local Char = model:Clone()
    local Pos = player.Character:GetPrimaryPartCFrame()
    local Schar = game.StarterPlayer:FindFirstChild("StarterCharacter")
    game:GetService("StarterPlayer")
    player.Character:Destroy()
    player.Character = Char
    Char.Parent = workspace
    local p = Char:Clone()
    p.Parent = game.StarterPlayer
    Char:SetPrimaryPartCFrame(Pos)
    Schar:Destroy()
    wait(30)

end)

If your projectile system is working then you first need to check if it is a player or not.

local Projectile = script.Parent
local Heartbeat = game:GetService("RunService").Heartbeat

while Heartbeat:wait() do
    Projectile.CFrame = CFrame.new(Projectile.Position, Projectile.Position + Projectile.Velocity)
    Projectile.Touched:connect(function(hit)

        local Player = game.Players:GetPlayerFromCharacter(hit.Parent)

    if not Player then return nil end

        local Model = game.ReplicatedStorage.Morphs.SweetCheeks

        local Event = game.ReplicatedStorage.Events.Morph

        Event:FireClient(Player, Model)

    end)
end

If the player variable wasn't an actual player, then the script will still run until there is an function, or property you want to change about the player. Or in all together anything that needs the player for some reason.

Sometimes functions return nil for a reason. They want to indicate that the following value isn't not found or returned.

If I didn't provide a right answer or it still does not work you can ask me