Oh, I see, interesting.
Since the Workspace is being referred as ‘workspace’ in that error (like, not capitalized), I don’t think it’s referring to the proper Workspace Service (game.Workspace/game:GetService("Workspace")), rather it must be referring to the script’s workspace (environment) or something like that.
After some investigation, this guy solved their instance of the issue by not saving their GUI instance to a variable and using that variable, but directly referencing it through an ancestor of it, so most likely it’s the script’s environment:
Hope that helps, but yeah, I agree it’s a weird error (and message), haha.