When in Print Module, and I click on media type under Print settings, Lightroom 6 crashes. I recently downloaded new print drivers for my Epson 3880 and I'm wondering if this is part of the problem as this is when the problem started to occur. Also just upgraded to Mac Sierra on my iMac
You can try few things to fix this issue.
1. Reset Lightroom preferences back to default, refer: Preference file and other file locations | Lightroom CC and 6
2. Uncheck the GPU from the preferences > performance tab > restart.
3. Install the updated printer driver from the manufacture website.
Let us know if that helps.