To avoid conflict between Anchor's invoice number sequence and QuickBooks Online's invoice number sequence, the custom transaction numbers setting in QuickBooks Online should be turned off.
To turn it off:
Log in to your QuickBooks Online account
In the top right corner, click Settings ⚙
Select Account and Settings.
Select Sales.
In Sales form content, select Edit ✎.
Turn Custom transaction numbers off.
Select Save, then select Done.
Addressing Existing Conflicts
If you've had 'Custom Transaction Numbers' enabled previously and created an invoice with Anchor sequence (ANC+Number), QuickBooks may continue this sequence even after disabling the feature.
To reset the sequence, perform the following steps:
Go to Settings > Account & Settings > Sales and enable Custom transaction numbers (set to on).
Create a dummy invoice: Navigate to New > Invoice and check the invoice number. It should appear as ANC+Number.
Modify the invoice number to either remove the 'ANC' prefix or adjust the numeric sequence based on your preference.
Complete the dummy invoice details (you can set the amount to $0) and click Save.
Go back to Settings > Account & Settings > Sales and disable Custom transaction numbers (set to off).
From the next invoice onwards, QuickBooks will follow the numerical sequence for manually created invoices.
Optionally, delete the dummy invoice.