Precise Number For Vector3?

Alright, well it's quite simple to solve the issue you've ran into. Let's say that x1 is 5, y1 is 3, and z1 is 7. What you're doing is:

Part.Position = (x1 + y1 + z1)

That's the same as doing:

Part.Position = (5 + 3 + 7)

and finally:

Part.Position = 15

Position requires a Vector3 value. What you're doing is just adding three numbers which isn't a Vector3 value. Here's your script fixed:

Player = script.Parent.Parent.Parent.Parent.Parent.Parent
Mouse = Player:GetMouse()


function Build()
    if Mouse.Target.Parent.Name == "Cell" then
        if Mouse.Target.Parent.Shrine.ShrineOrb.Owner.Value == Player.Name then
            print ("This Is Your Base,Building...")
            local Part = Instance.new("Part", game.Workspace.BuildenBlocks)
            Part.Name = "BuildenBlock"
            local MouseCFrame = Mouse.Hit
            print (MouseCFrame.p)
            Part.Anchored = true
            local x1 = math.floor(MouseCFrame.x)
            local y1 = math.floor(MouseCFrame.y)
            local z1 = math.floor(MouseCFrame.z)
            print (x1)
            print (y1)
            print (z1)
            Part.Position = Vector3.new(x1,y1,z1) -- That creates a new Vector3 value and sets the x value to x1, y value to y1, and z value to z1.
            Part.Rotation = Vector3.new(0,0,0)

        else

            print ("This Is Not Your Base,Build Cancelled")
end
end
end
Mouse.Button1Down:connect(Build)

Hope I helped! :)

On line 20 you are trying to set the position of a part, but you forgot to add Vector3.new before (x1 + y1 + z1) also, if you are setting values, you dont add them together you put commas inbetween like so

Part.Position = Vector3.new(x1,y1,z1) -- If you try to add them together, it is not a valid position as it doesnt say where x is, where y is, and where z is