There are two potential reasons for this:
1. Custom Transaction Numbers for your QuickBooks Online (QBO) invoices are not supported through QBO's API. When this setting is enabled in your QBO account, invoices will still be generated but you will not see an invoice number either in your sales → invoice grid view, or on the invoice itself.
You will need to rely on QBO's system generated invoice numbers when using Practice Ignition and QBO together. The system generated invoice number is simply a sequential numbering system.
Please note that disabling Custom Transaction Numbers should not affect QBO's ability to generate invoice numbers, either manually or automatically (via PI).
Turning off custom invoice numbers in QBO allows the PI -> QBO integration to work properly and raise invoices automatically whether it’s for a one time or recurring billing type - without affecting your ability to raise invoices manually. QBO will use the system generated invoice numbers to assign invoice numbers automatically.
If you are not sure if you are using custom transaction numbers:
Log into your Quickbooks online account
Navigate to settings → company settings → sales
Here you will see if Custom Transaction Numbers are enabled:
2. If you are not using Custom Transaction Numbers, but an invoice number is not displaying on your invoices, this is in fact a known issue to Intuit, which appears to have been resolved.
If you are still experiencing this issue, follow the instructions within the comments of this Intuit Community thread: