Unable to install SQL server management studio express

Run "Programs and Features" from "Control Panel" and scroll down, after everything loads (takes some time for the list to populate), to all-things Microsoft. If "Microsoft SQL Server" is the pre-fix to any of the numerous SQL likely-installeds, find any/all that you wish to uninstall and right-click to see what operations are available. Typeical ones are "Uninstall", "Change", "Repair", and I recall there are others as well. If you've got an option to "Repair" do that one first. Next try "Change" if it's there; just to see what you're up against ... if something is indeed installed.

I'm looking at your other recent post and I get the feeling that you've been looking at the list of GREEN checkmarks and RED exs that are displayed at the end of the NEW INSTANCE installation wizard. If your system is not meeting a particular requirement to upgrade/update a previous installation (versions are varied but this wizard refers to the version that actually got installed at some point) you'll be unable to proceed. As my memory serves me, clicking on any of the HELP links during that run to install should redirect you to MS docs about the particular section of the requirements that it's complaining about. Read carefuly. Everything.

And finally, if these two things fail just UNINSTALL all "Microsoft SQL Server" installations and start all over.