Thanks for connecting with us here. QuickBooks is designed to be an easy-to-use program. It's vital you're able to use the app without running into issues. Rest assured, I'll be happy to help so you're on the right track.
Based on what you've described, I highly recommend uninstalling and reinstalling the app. This is a great way to troubleshoot issues that may be affecting the app. By reinstalling the app, this ensures you're on the latest version of the mobile app. I encourage you to try this out and let me know what happens.
You're more than welcome to keep me posted in the comments below. I've got your back.
Thanks for reaching back out here and providing an update of your situation. The QuickBooks Online app does have some limitations and this would be why you're not able to select the Inventory asset in the category section. I recommend logging into your QuickBooks Online account from a web browser. This will allow you to complete your QuickBooks goals!
Feel free to let us know if this has been solved. Your success is important and we're always pleased to help!