This user posted what he did to pay for Plus now to unlock prior years. So try this.......
"After signing in, I clicked "Help" in the top right corner.
On the right side of the screen, a window opened up with a search bar, FAQ's, and at the bottom "Contact us"
I clicked "Contact us" and it said that I needed to purchase Plus to be able to talk.
I believe "purchase" or "Plus" was a hyperlink which I clicked and was taken to a page to confirm my payment info. After I did that, I had access to my previous year tax returns. "
That might only work during business hours M-F 5-5pm Pacific time (8-8 Eastern)