Connection Error Detected on Mac OS X El Capitan 10.11.6

I get the following error message every time I try to open an ebook (.acsm file) on my Mac computer running OS E El Capitan using version 10.11.6:

Digital Editions could not connect to the fulfillment server. Please make sure you are connected to the internet.

I AM connected to the internet. I have tried (several times) restarting the computer, opening and closing the program, deleting and re-installing the computer, etc. PLEASE HELP!!