MacOS – OS X will not update past 10.11

I am currently enrolled in the public beta for OS X.

I have OS X El Capitan 10.11 beta (15A278b). When I sent my feedback through the Feedback Assistant, I could not even select this build in the dropdown (I could only choose between 15E65 and 15F18c).

I have not received any updates to the later versions of OS X. App Store currently has no updates for my machine (everything is apparently up to date).

I thought about opting out of the public beta, but the option does not appear in System Preference > App Store (the section with the "Change" button is missing).

I downloaded OS X 10.11.4 from Apple's web site, but it tells me "OS X El Capitan Update can't be installed on this disk. This volume does not meet the requirements for this update."

I did not find someone else with the same issue. I want to avoid having to restore my Mac.

I'm on a 15-inch Retina MacBook Pro (late 2013).