Formula checkbox not working for more than one parameter

From a quick glance you're using & instead of 'and'. 'and' syntax is how Quickbase will combine conditions, where as & is how you combine text values. Same thing goes for 'or'. If you update your formula as follows you should be good.

If([Duration-Delivered - Created]<Days(30)
and Contains([All Session Modalities Next Session Date NOT CANCELLED or DOWNSIZED],"In-Person"), 
true)