Lots of potentially useful answers here, but none of them were worked for me.
On my Mac, I have both Android Studio and Visual Studio installed. One would hope that they would automatically look for an Android SDK in the same place, but no, not necessarily.
In Android Studio, go to Tools -> Android -> SDK Manager. In the window that pops up, make a note of the Android SDK Location. It should be something like
/Users/[user]/Library/Android/sdk
Next, in Visual Studio, go to Tools -> SDK Manager. In the window that pops up, the path that should be selected on the left is Projects -> SDK Location -> Android. On the right, click the Locations tab. For me, the Android SDK Location path was set to something like
/Users/[user]/Library/Developer/Xamarin/android-sdk-macosx
I changed this to the path that Android Studio was using, and that fixed the problem for me.