Would suggest you to reset Lightroom Preferences and then, try again.
Also check if you are able to click on Page Setup button in the left column of Print module.
Refer this link for instructions to reset Lightroom Preferences : Resetting (or 'Trashing') the Lightroom Preferences file - Lightroom Forums
I'm having the same issue with OS X 10.11.4, LR 6.5.1, Epson 3880 LAN (driver 04/21/2016). Also installed in LR6 are Nik and ON1 10.5 plugins.
Resetting the Lightroom Preferences file didn't help.
Since the last time it worked, I installed the ON1 plugin and updated Lightroom from 6.5 to 6.5.1
Any other suggestion ?
I found the solution: uninstall then re-install the printer.