The "Gap in Sequence" warning in the invoicing dashboard indicates that there are missing invoice numbers in your sequence. This could happen if an invoice was deleted or if an invoice was created but not validated. Here are some steps to help you resolve this issue:

* Check for deleted invoices: Go to the Invoicing module and select "Invoices" from the main menu. Check if any invoices have been deleted and try to restore them. If you cannot restore them, create a new invoice with the missing number to fill the gap.

* Check for unvalidated invoices: Look for invoices that have been created but not yet validated. These invoices might be drafts or cancelled. Either validate these invoices or delete them if they are not needed.

* Check if the invoice sequence is correct: Verify that the invoice sequence is correct and follows the expected pattern. In your case, it should follow the format INV/2324/23001, where "2324" represents the current financial year and "23001" is the invoice number. If there is an error in the sequence, you can update it in the "Sequence" section of the invoicing settings.

* Reset the invoice sequence: If you have resolved any missing invoice numbers and the warning still persists, you can reset the invoice sequence to start from the next available number. To do this, go to the "Sequence" section of the invoicing settings and click on the "Reset to Next Number" button. This will update the sequence and should resolve the warning.

By following these steps, you should be able to resolve the "Gap in Sequence" warning in the invoicing dashboard of your Odoo16 instance.

To fix this issue, you can activate developer mode and go to the invoices in the list view. From the header of the table on the checkbox at the left, check all the invoices and then go to actions and select the option "Resequence"
Click here to read more specifications: https://odootricks.tips/sequence-document-numbering/