No "Add Plan to Outlook Calendar" option

Ed Hansberry Tormod Solem Slupphaug dhood82 alloyd 

If this option is enabled in your tenant and you're still not seeing it in your plan options, make sure a group owner has published the iCalendar feed within your plan settings! 

Here's a help article on how to do this: https://support.office.com/en-us/article/see-your-planner-calendar-in-outlook-5dcccce5-2750-49b5-991b-1837379d96c7

Once the iCalendar feed is published, all members of the group should be able to see the option within Planner. 

Hope this helps!