The following projects do not have a valid ProjectGuid and were not built using a valid solution (.sln) thus will be skipped from analysis

As dotnet core projects (.csproj) will not have <ProjectGuid>...</ProjectGuid> tag specified in the default template this needs to be manually added.

So you need to edit the .csproj file like this:

<PropertyGroup>
  <!-- other properties here -->

  <!-- SonarQube needs this -->
  <ProjectGuid>{E2CEBBAF-6DF7-41E9-815D-9AD4CF90C844}</ProjectGuid>

Make sure to place your own GUID inside the <ProjectGuid>...</ProjectGuid> Tag