Firstly did you confirm that NET 4.8 isn't already installed on the machine? It is possible an application already put it there.
Next, run the (NET Framework Repair Tool](https://learn.microsoft.com/en-us/dotnet/framework/install/repair).
After that collect the setup logs for the installer and look to see what is failing.
If nothing stands out, and you've done the standard stuff like disabling AV and rebooting and all that then consider installing an earlier version first (say 4.7.2) and see if that works. If it does then the installer may be corrupt so download it again and run it.