"Position is not a valid member of Workspace?"

First you should be using local variables where possible.

local part = Instance.new("Part", game.Workspace)

Secondly the Parent argument for Instance.new should not be used. This is because of network replication as when the instance is part of the game the changes made to this instance would be replicated. Setup the instance then parent it to avoid needless network replication of changes.

Lastly your error is caused by you assigning part to hold a reference to the scripts parent which in this case is the workspace. I think you have mixed up how to set the parent and using the parent argument for Instance.new.

part = script.Parent -- this sets the variable part to the scripts parent causing your error

Putting this all together.

local part = Instance.new("Part") -- create instance no parent arg meaning the parent is nil

-- setup the part
part.Position = Vector3.new(27, 0.5, -3)

-- parent the part to the game
part.Parent = workspace

Hope this helps.