Where do I have to activate Bill Number?

Let me provide some details about how a bill works, Aym.

Currently, QuickBooks Online (QBO) doesn't automatically generate a bill number. You'll have to enter them manually every time you record a transaction. You'll want to send a request to let our product team know that being able to automatically generate a bill number is helpful for your business. 

To do so, follow the steps provided below:
 

1. Go to the Gear icon, then Feedback.
2. Enter your comments or product suggestions. Then select Next to submit feedback.

This goes to our Product Development team to help improve your experience in QuickBooks Online. You can track feature requests through the QuickBooks Online Feature Requests website.

Furthermore, learn how to record bills one at a time or record multiple bills for multiple suppliers at once and how to record their payments. I've added this article for more information: Enter Bills and Record Bill Payments in QuickBooks Online.

You can always get back to us if you have any questions about creating a bill. We're always right here to help you. Have a nice day!