Does Airport Express (sec. gen. 2) connect?
Does Airport Express (sec. gen. 2) connect to iMac w/ OS X El Capitan 10.11.4? Mine does not while both my phone 5c and iPad mini do connect to the Airport.
iMac (21.5-inch, Late 2013)
It should.. I see no reason for it to fail.
Try doing a factory reset of the Express.
Redo the setup from the El Capo Mac airport utility.
Use short names for everything, no spaces and pure alphanumerics. Passwords the same but can be 8-20 characters or more depending on your current level of paranoia.