Package 'Microsoft.Bot.Connector.Teams 0.9.0' was restored using '.NETFramework,Version=v4.6.1' instead of the project target framework '.NETCoreApp,Version=v2.1'
You're probably getting this warning if you're using a higher version of Visual Studio, like Visual Studio 2019. Regardless of this warning, you should be able to build the solution without any problems.
Package 'Microsoft.Bot.Connector.Teams 0.9.0' was restored using '.NETFramework,Version=v4.6.1' instead of the project target framework '.NETCoreApp,Version=v2.1'