It costs 19.99 to upgrade your 2018 Free Edition return to PLUS.   "Upgrade" is not a button, but is a tab link in the left menu column of an open return.

• So your 2018 online TurboTax return has already been filed?
• And when you click "Add a State", does your 2018 return open up? 
• After it opens, in the left menu column is there no Upgrade link at all?  Be sure your return is fully open.  You may have to proceed a couple screens then check the left menu column again.    Are you seeing other items in the menu such as My Info, Federal, State, Tax Tools, etc.?

In the interest of time, here's some more info.

Be sure the 2017 and 2018 returns are in the same account.

Once you open the 2018 return with "Add a State", then in the left menu column click on Upgrade, then choose PLUS.  Do not upgrade any farther than PLUS, or you'll have problems.  Then here are the rest of the steps.

• Once you are in PLUS, then you need to pay.
• In the left menu column click on TAX TOOLS, then PRINT CENTER.
• At the Print Center choose to "Print, save, view THIS  YEAR's return."   (Yes, you say "this year's" because you are paying for the 2018 return to unlock the 2017 return.)   You don't actually have to print the 2018 return.
• It will ask you to pay.  If the Order Summary appears OK (PLUS is 19.99), then choose PAY NOW.
• You can pay by credit card or debit card (even though it only says "credit.")

NOTE:  If it should offer you the choice of paying later out of the Federal refund, that cannot be done for this purpose.  You have to choose PAY NOW.

There's also a free option if a return transcript will suit your purpose.   You may be able to immediately download one at the IRS website if you qualify to open an IRS account there.  Otherwise, to order a transcript to come by postal mail takes 5-10 calendar days.
http://www.irs.gov/Individuals/Get-Transcript