On8April2018 I purchased Self Employed Fed and State. Trying to download my return I get an error saying I can't view my return without purchasing an additional product

[DanPaul02]

If you did not purchase a TurboTax product for 2019, then you will have to purchase Plus to download your 2018

[VolvoGirl]

What year are you trying to get?  In April 2018 that would have been for your 2017 return.  You don't have to pay now especially since you already paid.  See this answer from Don.  Scroll down,  you may have to start a  2020 return for the prior years to show up.

https://ttlc.intuit.com/community/after-you-file/discussion/re-how-can-i-access-my-2018-tax-returns-...