@ee-ea Your link only points to re-installing. Other TT posts have much more details like "Run as administrator", making sure folder have full permission for SYSTEM, etc.. But I can say definitively that a re-install is not the answer. My install and re-install work fine. It is not until it runs the Update process that it fails. Her is what I found. Hopefully this will help others save hours of frustration like I did.
have tried every recommendation from TT support and MS installer error support. None worked. And this text from a MS site had me wondering.
"The drive that contains the folder that you are trying to install the Windows Installer package to is accessed as a substitute drive."
I don't like to install programs to my C drive. I have separate drive just for program installations. (It's a portion on the physical drive if that matters.) I have done this for 20 years. And never had a problem. But this year, I ma getting the 1603 error. First installation goes without a hitch. But subsequent updates fail. And the updates fail on previously installed version as well (2020 in this case). So I decided to try an experiment. I uninstalled my TT for Business 2021 software via Windows uninstaller. I then cleaned the registry of any remaining entry pointing to the Program Files location. I reinstalled TT for Business 2021. It still saw a version "installed". So I let it remove it. Then I re-installed it, allowing it to go to the default location. And now updates run without an issue.
I have done this same re-install procedure on 2 other version now TT 2020 and TT for Business 2020. Updates that failed before the re-install now run successful. So maybe MS changed their installer. But I do not have issues with the dozens of other programs I use. So the fix for this needs to come for TT development.
https://support.microsoft.com/en-us/topic/ccfda1f5-4601-0cbc-07b2-9ecc93f009be#:~:text=The%20drive%2....
