Do the following, then try running the installer again:
Issues like this can normally be solved with a Microsoft install/uninstall troubleshooter app that helps with this situation. The app is here:
https://bit.ly/2ViOQNjRun the app, It will do a detecting problems step. If that doesn't find anything, and it normally doesn't, then it asks if you have a problem installing or uninstalling an app. Select Uninstalling.
It will provide a list of apps. Select DaVinci Resolve, then Next. Then select "Yes, try uninstall".
Hopefully that will solve the issue for you. It should not impact any of your existing projects and settings