Hi there, Andre!
This happens when the Form Numbers option is not enabled on the invoice form style that you use. I can guide you to enable it, and you'll see the number on your actual invoices.
I've added a screenshot for your reference.
If you need anything else, please let me know. Have a good day!
Thanks for joining the thread, RogerM.
Aside from checking the Form numbers, we also need to make sure that you've selected the correct template. Create an invoice as usual. Click on Customise button at the bottom. Then, select your customised template. You can also follow these steps to make it default:
Let's perform these steps if what you'd like to achieve is to show the Invoice no. field when creating it. This field won't appear on the invoice page if this option isn't turned on. Though, it'll still show when previewing it since QuickBooks will automatically generate an invoice number.
Also, let's check if your Adobe Reader/Acrobat is updated. It can cause errors when previewing a document if you're using an old version. You can check this article as your guide on how to update, repair, or re-install Adobe Reader/Acrobat.
I'd suggest contacting our Customer Support Team if there are no changes. They'll be able to check the cause of it.
You can check this article if you need help with recording customer payment.
Hope this helps. Reply to this thread if you need further assistance from me.