If PLUS is grayed out, that means you are already in PLUS, and that's what your image confirms.
You said you "signed up" for PLUS a few weeks back. But have you actually paid for it? You have to pay the 19.99 by credit/debit card to actually unlock prior returns in the same account. If you haven't yet paid for it, we can tell you all the steps to pay for it.
And are the 2016 and 2018 returns in the SAME account?
Have you already filed the 2018 return and upgraded to PLUS after filing? Or is the upgraded 2018 return still in preparation?
