This is the reason for the problem. The MonoGame extension is not compatible with the latest version of Visual Studio for Mac. There are 2 possible solutions for this.
Either downgrade Visual Studio for Mac by uninstalling the current version and then get the older one from here : https://visualstudio.microsoft.com/vs/older-downloads/ After that, continue to follow the installation instructions in the MonoGame documentation here : http://www.monogame.net/documentation/?page=Setting_Up_MonoGame_Mac
If you are not in hurry, you can wait for MonoGame to be updated to support the latest version of Visual Studio for Mac.