Not able to add serial no in invoice

Hey there, @info-saffronchem.

I'll help you accomplish your task in QuickBooks Online (QBO). 

Are you trying to set your own numbering preference in QuickBooks? If so, you can use the Custom transaction numbers feature. This adds an Invoice no. field where you can enter any number or letter format you want. Let me show you how:

1. Go to Settings ⚙ and select Account and settings.
2. Click Sales, then select anywhere within the Sales form content section.
3. Turn on Custom transaction numbers.
4. Select Save and click Done.
5. Go back to Sales and select All Sales.
6. Open an invoice or any sales form.
7. In the Invoice no. field, enter the custom number or letter sequence you want to use for your forms. The sequence you set here applies to all sales forms.
8. Click Save or Save and close.

Once you have set up your custom invoice numbering, turn off the Custom transaction numbers feature. This will help prevent accidental changes to your numbering. 

Turning this feature off only means the Invoice no. field is hidden from your sales form. Though, invoice numbers are still printed for your customers. Here's how:

1. Go to Settings and select Account and settings.
2. Click Sales.
3. In Sales form content, select Edit ✎.
4. Turn Custom transaction numbers on or off.
5. Click Save, then select Done.

You can also add custom fields to your sales forms.  This gives you the flexibility to add custom data and drive insights through detailed reports. That way, you can specific info to your invoices including serial numbers. 

Please know that you're always welcome to post if you have any other concerns. Wishing you and your business continued success.