I am trying to upgrade to plus to access my previous documents and it just sends me back to my welcome screen. I have tried multiple times and I cannot go any further.

[Critter]

Until the 2019 Online version goes live sometime in December you will not be able to unlock a prior year return ....

Access to prior year returns.

https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-access-my-prior-year-return/00/270...

You can get a free transcript from the IRS:   

https://www.irs.gov/individuals/get-transcript

How do I get a copy of a return I filed using online TurboTax?

https://ttlc.intuit.com/questions/1900748

How do I get a copy of a return I prepared using desktop TurboTax?

https://ttlc.intuit.com/questions/1900639

How to upgrade to PLUS

How do I purchase TurboTax PLUS so I can get my .tax file?

In TurboTax Free Edition, you can purchase PLUS to unlock your prior-year .tax file.

The exact instructions will vary depending on the status of your 2019 TurboTax Online return (select one):

• I haven't started a 2019 return yet
• I started a 2019 return but haven't filed it yet
• I've already filed my 2019 return

Note:  While you still can, print a copy of your current year tax return and save it as a pdf, too.  That way you will have it when you need it for school, a loan, etc. without going through a lot of aggravation to get it.

[mesquitebean]

User justin_michael91 in the thread linked below said he was able to pay for PLUS this way:
  Right after signing in, clicking on HELP in the top right, then scrolling way down to CONTACT US.  Then following the prompts, entering a question, then choosing to phone Support.   It required him to pay for PLUS to get phone privilege, and that unlocked the past returns.

Whether that will work for everyone or not, I don't know.   Here is the thread where he described what he did:

https://ttlc.intuit.com/community/after-you-file/discussion/re-prior-year/01/951264#M195037