After installing Inventor 2020 and then spending several hours of trying to revert back to 2019, my suggestion would be not to install Inventor 2020 until Nastran 2020 is available.
I made the mistake of installing Inventor 2020. I then tried to go back to 2019 when I realised Nastran In-CAD was not available in 2020. Also, my Sim Mech 2018 would not work from Inventor 2020.
My Nastran In-CAD 2019 will not now work - or at least the screen freezes when I click the edit button. The only way out is control-alt-delete. I have done each of the repair install options. The next option is to do a full delete everything reinstall.
(Something I was intended to do anywary because the default Autodesk install puts the install files on C:\ without any option to change drive as is the case when doing an install after a file download. c:\autodesk can knock a big chunk out of one's SSD. I therefore never use the default install if I can remember).
Running Sim Mech 2018 as standalone after reverting to Inventor 2019 failed because it not able to register. I assume that the failure was either fall out from the Inventor 2020 install or a coincidence. A repair install seemed to fix it.