I have installed .Net Framework 4.8 and confirmed the same in the registry entry.
I am still not able to see the same in Target Framework dropdown in project properties tab.
Microsoft have made it very confusing with the different Project templates to create any 2.0 to 4.8 .Net Framework projects
The following template sounds plausible:
Note how there are only two options.
The correct templates to look at :

Contain "(.Net Framework)" within the title:
.NET Framework 4.8support is checked either inIndividual Itemsor in the payload section you wantuse the Installer