Sorry, but you can't expect TTX to store your files for free for years.
If you download your tax returns as a PDF file EVERY YEAR, right after you file has been accepted, then you never have to worry about it. They are always available in the year you file..up until mid-October. Then Back-Them-Up off computer too. (Hundreds of people have computers crash with no backup every year...then what?)
This doesn't solve the problem of being unable to access previous year returns. It will not allow me to upgrade to view them unless I have weeks to spend waiting for the IRS.
The exact instructions will vary depending on the status of your 2018 TurboTax Online return (select one):
IF you used the "FREE" version in a prior year, then you must upgrade...and PAY for PLUS (or higher version) in the exact-same UserID account that you used in the prior year you are attempting to print now.
IF you switched accounts between the years (should not do that unless you divorce, separated, or get married between tax filings.....or start doing MFS tax returns)....then you must have a paid-for 2018 tax account (PLUS or higher), and then you have to call Customer Support to get the other account unlocked:
https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632
Every year, before mid-October, print a copy of your current year tax return and save it as a tax file and pdf, too. That way you will have it when you need it for school, a loan, next year’s tax return, etc. without going through a lot of aggravation (and expense) to get it.
https://ttlc.intuit.com/questions/1912544-how-do-i-save-my-2018-turbotax-online-return-as-a-pdf