QB Issue Resolution:

This is a preference that you can turn on or off. If you click the gear next to your company name at the upper right then go to Company Settings. Click on Sales Form Entry on the left then Custom Numbers. Once checked and Saved, the field will be open for you to change.

Resolution for Issue 'How do I restart invoice numbering' available: Yes (Solved).
Source: Intuit Community forum.

How do I restart invoice numbering: 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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