Thank you @gustavo.cervelin. I think I’m getting closer 
nuget.org is enabled
I uninstalled excel and mail activities. Before doing that, they had the red (!) - obviously not working properly. So, I uninstalled them. When trying to find them to install them again, I get the “no package found” message.
I downloaded the packages, I put them into the correct folder, in my case: C:\Users\florin.ciutacu\AppData\Local\Programs\UiPath\Studio\Packages ,restarted Studio, and try to add them. Studio cannot find those 2 packages. I get the “no package found” message again. How can I add them? 