How can I begin 2019 tax return...it only lets me start in 2021... I did use turbo tax for 2018...?

To do a prior year return you have to buy the Desktop program here,

https://turbotax.intuit.com/personal-taxes/past-years-products/2019/

You will need a full Windows or Mac to install it on.

If you paid for the Online version you can ask for the 2019 Desktop program.

If you have a simple return and want to file 2019 for free you can fill out the forms by hand.  Here are some basic forms.....

Here is the 2019 1040 return

https://www.irs.gov/pub/irs-prior/f1040--2019.pdf

or if you want bigger type use 1040SR for Seniors,

https://www.irs.gov/pub/irs-prior/f1040s--2019.pdf

2019 Instructions https://www.irs.gov/pub/irs-prior/i1040gi--2019.pdf

2019 EIC and Tax Tables https://www.irs.gov/pub/irs-prior/i1040tt--2019.pdf

2019 Schedules 1-3

https://www.irs.gov/pub/irs-prior/f1040s1--2019.pdf

https://www.irs.gov/pub/irs-prior/f1040s2--2019.pdf

https://www.irs.gov/pub/irs-prior/f1040s3--2019.pdf

Don’t forget your state.  And you will have to print and mail your returns.  So be sure to attach copies of your W2s and any 1099s that have withholding on them.  You have to mail federal and state in separate envelopes because they go to different places.  Get a tracking number from the post office when you mail them for proof of filing.

If you used the Online version for 2018 you can try to download the 2018 data file ending in .tax2018 to transfer into the Desktop program.   

How to access prior online returns

https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-access-my-prior-year-return/01/27010

You have to log in to the same account.   You need to start entering some basic Personal Info in 2021 for the side menu to open up. Just continue a little ways into 2021.