The 'send premium text message' flag may have been set to 'never', Please locate in Android Settings > App Manager > Textra SMS and set it to allow.

Or

Android Settings > Apps > (in the top right of the screen) > Special Access
> Use Premium text message services.

NOTE: For Android M, there is no way to reset this, it's an Android bug. See https://productforums.google.com/forum/m/#!topic/nexus/WjO0Sz5N-ew