If you've expanded fully and covered all your walls with that wallpaper, and THAT wallpaper is the only option to get by the goal, you'll have to use Gems to skip it. If you haven't fully expanded, you'll need to expand enough to purchase more of those wallpapers. From what I've read of posts with the Thanksgiving goals, TL will not help you SKIP that goal due to purchasing wallpaper/floor tile ahead of time. Many of TL goals are NOT retroactive, so you're better off NOT purchasing ANY decor until after you've completed the goals.
You can email
support@teamlava.com and advise them of your situation, but I would expect them to tell you that you'll need to skip with gems or expand enough to purchase more wallpaper. Sorry.