Hi @morganpuff! This error isn't related to your data or qiime demux summarize, it's a system configuration issue with your matplotlib installation. matplotlib's default configuration requires a display, which you don't appear to have (this is common if you're logged into a server and don't have an X server and X-forwarding set up).
This topic will show you how to configure matplotlib to work in a headless environment. Get in touch if you have any issues!