Expected 'then' ,got '=' ?

The problem is that = is the assignment operator, but the equality operator is ==:

door=script.Parent
turn=1

function move()
    wait(0.1)
    if turn == 1 then
    door.Position = door.Position + Vector3.new(0,-1,1)
    door.Rotation = door.Rotation + Vector3.new(90,0,0)
    turn = 0
    end
    if turn == 0 then
    door.Position = door.Position + Vector3.new(0,1,-1)
    door.Rotation = door.Rotation + Vector3.new(-90,0,0)
    turn = 1
    end
end

door.ClickDetector.MouseClick:connect(move)

There are also a number of ways to improve this, such as using CFrames for moving the door rather than Position and Rotation. Additionally, you can (and should) use booleans rather than numbers, for example:

door=script.Parent
turn=true

function move()
    if turn then -- This is the same as 'if turn == true then' and is generally preferred.
        door.Position = door.Position + Vector3.new(0,-1,1)
        door.Rotation = door.Rotation + Vector3.new(90,0,0)
    else
        door.Position = door.Position + Vector3.new(0,1,-1)
        door.Rotation = door.Rotation + Vector3.new(-90,0,0)
    end
    turn = not turn -- This can be used to invert a boolean.
end

door.ClickDetector.MouseClick:connect(move)