I initially attempted to perform a Flexcell on 10.3 Rio's release date, and needless to say it didn't install correctly. I came back a few days ago and attempted to re-install, and it appeared to install, right up until the point when I received an error indicating that it couldn't find MSBUILD. Now I'm attempting to uninstall it and reinstall it via the TMSSubscriptionManager2, but now TMS FlexCel doesn't have icons under install or under download under the Subscription manager. I deleted the directory where it was installed but that didn't seem to help.
Sadly when you install Rad Studio 10.3 from the ISO, the paths are setup wrong and it won't find MSBuild. You need to either fix rsvars.bat, or install with the Web installer. All tms setups (and likely all others too) are broken in 10.3 when installed from the ISO.
Please read more here https://www.tmssoftware.com/site/blog.asp?post=507
Once you fixed rio, the simplest way is to reinstall from the Setup.exe directly. Once it is installed the tms sub manager will detect it, and from then on you can keep using the sub manager to install newer versions