Can't send mms with Hangouts
On many devices, messaging apps (even those with full access to SMS) are unable to send MMS messages unless they are set as the default SMS app. 

This same issue affects to me on my Nexus 6P: my default SMS app is the Messenger app, and even though other messaging apps on my phone (Textra and Pulse) have access to SMS &amp; MMS, neither can send MMS without being set as the default SMS app in Settings. Both of those apps prompt me to set them as the default SMS app when attempting to send MMS, I'm surprised that Hangouts does not.

So, to fix the issue: go to Settings -> Apps -> gear icon in the top-left -> SMS app -> Hangouts. Or, alternatively, search through Settings for "Default apps" or "SMS app".

tl;dr: Setting Hangouts as your default SMS app will fix the issue.