What won't start? Did you start online and trying to switch to the Desktop program? The program won't start or you can't open your return in it?
Do you have the tax file ending in .tax2020? Or are you trying to open the pdf file?
If your .tax2020 File has any state returns in it you need to first download the state program.
After you get the program installed the first thing to do before you open your tax return is to update the program and install any state programs you had. Then open your file. So you first might need to start a fake return to be able to download the state program (go to FILE - NEW TAX RETURN)
Then go to FILE -Open and find your return.