I know how important it is to have this feature in QuickBooks Online mobile app. However, the project feature is currently unavailable in the mobile app.
As a workaround, you can set it up through a browser using a different device like a laptop or a desktop.
Here's how:
Go to Settings ⚙, then choose Account and settings.
Select the Advanced tab.
Look for the Projects section and select Edit ✎ to expand it.
Turn on Organise all job-related activity in one place.