QB Issue Resolution:

Welcome to QuickBooks Online! To change an invoice number, we’ll need to add a Preference: click on the Gear (upper right), Company Settings, under the section called Sales Form Entry, choose Custom Numbers. Now you’ll be able to change the last Invoice number so the next one starts with the correct number. I hope that helps, and once again, thank you for choosing QuickBooks Online!

Resolution for Issue 'Enter a new invoice number' available: Yes (Solved).
Source: Intuit Community forum.

Enter a new invoice number: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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