I want to add Vat Number In Purchasing Invoices as i add the custom field but its showing only in sales Invoices

Thanks for dropping by, MUDASSIR. I'll give you clarifications why does VAT Number can only be added in Sales Invoice.

QuickBooks Online currently offers more customization options for sales forms than for purchase forms. QuickBooks Online currently offers more customization options for sales forms than for purchase forms. While this limits the ability to add VAT numbers directly to Purchasing Invoices, it presents an opportunity for future enhancements to improve purchase form functionality and provide users with more comprehensive VAT management across all transaction types. Thus, the reason why we aren't able to add VAT number in your Purchasing Invoices. 

However, as a workaround, we can manually add a VAT Number by adding a column to your Sales Invoice.

Here's how:

1. Go to the Gear icon, then select Custom Fields
2. Click the Add field, and then enter VAT Number inside the box.
3. Tick the beside All Sales Forms, as well as the Print on the form radio button to reflect in your Purchasing Invoices.
4. Once done, click Save.

Afterwards, you can now try to check your Sales Invoice and from there, you'll be able to manually add VAT Numbers as it will also reflect to your Purchasing Invoice.

Moreover, here's an article you can read to learn more about how to set up custom fields on Sales forms: Add custom fields to sales forms and purchase orders in QuickBooks Online.

Furthermore, you can also read this article if you wish to manage invoice payments in QBO: Record invoice payments in QuickBooks Online.

I trust this explanation and workaround will assist you in managing your VAT numbers effectively, MUDASSIR. Please don't hesitate to reach out by revisiting this thread if you have more questions with regard to your Purchase Invoices. We're always here to help.