Just a comment. It's a know issue with Mac Computers since about 10.2 Wolfram told me they are aware of the issue.
On a Mac, commands that access "Entity" values from the web also do not work. (Especially accessing Geo' Function.
The solution was to reset the Paclet data a few times, and reboot the computer before it would work.
It took a while to get it reset on the Mac so it would access the web data correctly.
Just some additional versions...
UnitConvert[Quantity[12, "Meters/Second"], "Miles/Hour"]
Quantity[37500/1397, Miles/Hours]
UnitConvert[Quantity[12, "Meters/Second"], "mph"]
Quantity[37500/1397, Miles/Hours]