Hello trr2,
Thank you for posting about adding a bill using the QBO mobile app. At this time, this feature is only available in the web interface. If you always use your iPad or phone, you can sign in with a mobile browser so you can make bills.
I'd like to add this resource for your additional reference: QuickBooks Online iPad: Frequently Asked Questions (FAQ).
The QuickBooks Team is always available for any other questions you may have. Take care and have a good one!