The problem was reported several times on several forums: I searched all of them and did not find an answer that worked in May 2017, now that the Xamarin Universal Installer is no longer provided to Xamarin.com, only Visual Studio. Most of the answers I have depends on two things: one that loads Android sdk, ndk and Java jdk and communicates with them in the Tools Project → Android option. Although a suitable answer does not seem to be a problem for those who had this on a PC like me. I also had Visual Studio installed, but I wanted to see Xamarin Studio. Other parts of the answers I've seen mention the use of the Xamarin Universal Installer. Only manual installation of Xamarin Studio (through its .msi files) is now possible taking into account the offers of Xamarin.com.
However, even uninstalling and reinstalling Xamarin Studio, as recommended by the forum, did not seem to solve this problem. Normal uninstallation using the Xamarin Studio control panel does not delete the user's local information. It is also related to Visual Studio. As far as I understand, this is a mix between Visual Studio in its latest forms and Xamarin Studio in its early forms on the same Windows PC, which causes some configuration problems.
So, the solution looks like if you want Xamarin Studio to work on PCs with the target Android platform, you should start from a clean PC and start working with Android sdk and ndk and Java jdk, and then install Xamarin Studio without first installing Visual Studio. Hopefully though Visual Studio should be good enough.