Selecting a Humanoid rig for an imported character mesh means that Unity will automatically try to configure your mesh in a special way that Mecanim requires. It does this using a data structure, called an Avatar. When the autoconfigure process completes, your mesh will either pass or fail the autoconfiguration process. If the process fails, then you will certainly need to configure your mesh Avatar manually. To do this, click on the Configure button to show the Avatar editor. Unity will switch to a different mode, and it may ask you to save your current scene before doing so.
Remember that even if the process of avatar generation completes automatically, you may still need to manually tweak or change a character Avatar. For that reason, don't skip this section even if your character mesh gets imported without any issues.