I have a problem on the invoice no. It is not following as it was like the last invoice no I have was 8015 then next 8018 instead of 8016. How to fix?

Hi there, customer. I have some information about some factors that can affect the sequence of your invoice numbers in QuickBooks Online.

It's more likely that invoices were created and then deleted after invoice no. 8015, which explains why the invoice sequence was skipped.
 

To resolve this, check if the Custom transaction numbers feature is turned on. With this, we'll be able to override the invoice number sequence.

1. Go to the Gear ⚙ icon, then select Account and Settings.
2. Choose Sales tab.
3. In the Sales form content section, turn on Custom transaction numbers.
4. Click Save, then hit Done.
5. After that, open invoice no. 8018.
6. In the Invoice no. field, enter the custom number you want to use for your forms. The sequence you set here applies to all sales forms moving forward.
7. Select Save or Save and close.

To learn more about the customer transaction number feature, check out this link: Add custom transaction numbers to sales forms in QuickBooks Online.

I've also added this article as a reference once you're ready to receive customer payments: Record invoice payments in QuickBooks Online.

Let me know if you need further assistance with invoices. I'll be here to help. Take care!