Hi @Jenna_Shelton, thanks for posting your machine specs. It sounds like you are in fact running out of memory, which is a bummer. We have seen this with the SILVA database, it seems to need quite a bit more memory when compared to something like greengenes (FWIW, we develop QIIME 2 on machines spec’ed similarly to what you posted, and we have had the same problems crop up).
Moving forward, it sounds like you might need to run this step somewhere on a machine with more available memory. If you have an institutional cluster you could see if they could install QIIME 2 there for you (it is pretty straightforward). Another option, which might be quicker, is to launch one of our QIIME 2 Amazon Web Services instances (you will want to read up on AWS EC2 instances if you are unfamiliar with cloud computing, but basically you can rent some hardware from Amazon for pretty cheap, and we have ready-to-roll installations of QIIME 2 available there.
As a last-ditch effort, I am pinging @BenKaehler, the brains behind q2-feature-classifier, just to make sure there isn’t some other trick available. Sorry I don’t have better news for you 
.