Used online version for 2019 , now I can't open the tax form. What can I use to read the form?

In order to view the documents, you will need a PDF viewer installed. Once you have one installed you can refer to this Turbo Tax FAQ to show you how to access your return.
How do I access my prior year's return?

If you can’t access your prior year's return, you can request a copy of the transcript from the IRS.

What file did you download from online?  You should have a pdf file which any computer should open.   If you have the tax data file ending in .tax2019 you need that years Desktop program to open it.    You can call in and ask for it.  But they might not give it to you.

You could try the TaxPrinter service (not connected with Turbo Tax) to convert a .tax file to pdf at

http://www.taxprinter.com

OR get a copy from the IRS -

http://www.irs.gov/Individuals/Get-Transcript