Had the same type of problem, saw ‘no sdk found’ and ‘dotnet not a command’ in the logs even though I have a full install of visual studio. Installed a few versions of dotnet before realising it would only recognise exactly version 3.1.0.
There’s a link in the logs but it took me to 3.1.125 not 3.1.0 and didn’t work. In the same area on the microsoft website there is a full set of installers (runtime, sdk) for 3.1.0 that got my setup working.
I used this one:Download .NET Core 3.1 SDK (v3.1.100) - Windows x64 Installer
It says 3.1.100 but puts the runtime and sdk as 3.1.0 in the registry and was recognised immediately by ue5.