There's the problem: CS6 is not compatible with El Capitan. Every day we hear reports of problems with that combination.
Either roll back to a previous Mac OS version or upgrade to the InDesign CC.
I actually upgraded to El Capitan to try and fix the problem. I cannot use CC because I live in a rural area with satellite internet.
There is nothing about the CC versions of InDesign that really require any significant additional amount of internet access than the CS versions after you have downloaded and installed the software. The software doesn't run in the cloud, but on your system as it did in the past.
Really? I was under the impression the program operated in the cloud. Thanks for the information.