I'll take care of this query for you.
The option to track inventory within the QuickBooks Online (QBO) mobile app is unavailable at this time. A workaround I can provide is for you to open your mobile browser and log in to your QBO company from there. Another thing to note, please ensure the product(s) included in the invoice is an inventory item.
I'll share this article for your reference: Features of the QuickBooks Online app. It's a list of features available for the QBO mobile app.
Let me know if you have other questions in the comments below. I'll be sure to take care of it.