I frequently run into this issue when updating MonoGame. It might just be my environment, but I hope this helps someone!
Create a new nuget.config file in the project root (the folder where the .csproj file is located).
<configuration>
<packageSources>
<add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
</packageSources>
<packageSourceMapping>
<packageSource key="nuget.org">
<package pattern="*" /> <package pattern="dotnet-mgcb*" />
<package pattern="MonoGame.*" />
</packageSource>
</packageSourceMapping>
</configuration>
and then save it. Right-click that location to open a command prompt,
dotnet nuget locals all --clear
dotnet clean
dotnet restore
dotnet tool restore
And succeeded!
I would very much appreciate it if this could be handled by the system.