[ADVANCED] How to make an object appear in front of a player? Please help?

The problem is that you aren't using the right data types. You have not used Vector3.new() to put position or size. Do this instead:

local uis = game:GetService('UserInputService')
local wall = game.Workspace.MassiveMudWall.Part
local player = game.Players.LocalPlayer

local useStatus = false

local usePosition = player.Character.Position

uis.InputBegan:Connect(function(input)
     if input.KeyCode == Enum.KeyCode.E then
         print('Point1')
         local massiveMudWall = Instance.new(brick, game.Workspace.MassiveMudWall)
         massiveMudWall.Size = Vector3.new(31, 31, 2)
         massiveMudWall.Position = Vector3.new(-18.5, 15.5, (usePosition + 5))
         wait(5)
         massiveMudWall:Destroy()
     end
end)

To be honest. I'm surprised you did not notice any errors in your output.

Man, there is a lot much wrong with this, in a lot of different ways. First of all, Characters do not have a position value so line 7 would error. You should use the position of their head instead. Next, line 12 would not work because brick is not a type of instance. Third of all line 13 and 14 would not work because it should be vector3 values. And then on top of all of that, you should be using CFrame instead of Positions. Here is what the script should look like. Mess around with the CFrame value of line 15 to your liking.

local uis = game:GetService('UserInputService')
local wall = game.Workspace.MassiveMudWall.Part
local player = game.Players.LocalPlayer

local useStatus = false

uis.InputBegan:Connect(function(input)
    if input.KeyCode == Enum.KeyCode.E then
        if useStatus == true then
            useStatus = false
            print('Point1')
            local massiveMudWall = wall:Clone()
            massiveMudWall.Parent = wall.Parent
            massiveMudWall.Size = Vector3.new(31, 31, 2)
            massiveMudWall.CFrame = player.Character.Head.CFrame * CFrame.new(0,0,-30)
            wait(5)
            useStatus = true
            massiveMudWall:Destroy()
        end
    end
end)