To fix this error, you can manually add a NuGet package source in the Options window and setting it to the following path: C:\Program Files\Microsoft SDKs\Service Fabric\packages.
After adding the package source, you should be able to add Service Fabric projects to your solution without any issues. If you are still having trouble, you may want to try repairing your Visual Studio installation or reinstalling the Service Fabric SDK.
If this does not work, you can try adding an existing service by right-clicking on the Services node in the Solution Explorer in VS and selecting “Add Existing Service” or add additional services by selecting Add > New Service Fabric Service.
Let me know the results in the comments and I can investigate further. Comments are the quickest way of notifying the experts.