identifiers must be set for the customer in order to send e-invoices

Yes, that's correct. In QuickBooks Online (QBO), you need to set up identifiers like an email address or customer ID for a customer before sending e-invoices or electronic invoices, Amira.

You can simply enter N/A if you want to skip that section. 

To learn more about setting up e-invoicing in QBO, you can check out this article: How to set up e-invoicing in QuickBooks Online - Malaysia.

If you have additional queries about sending e-invoices, you can always come back to this forum.