My college professors said that an encyclopedia article can't be cited because it's a tertiary source of information. That's considered too far abstracted from the original source of the info to be used as a citation. Primary sources are original documents like the U.S. Constitution; secondary sources are books about the U.S. Constitution that offer some kind of analysis by professionals; tertiary sources are aggregate articles citing multiple analyses, aggregating them together like a Reader's Digest but not contributing further to the discussion like a viewpoint or stance or new info.

All of the above is explained in this article[1] (I just lazily searched for one).

Sources: 1: https://crk.umn.edu/library/primary-secondary-and-tertiary-s...