I just saved my TRN number in Account settings but the TRN number is not reflecting in my invoices

Welcome to the Community, @ffakhri.

Allow me to share with you some information on why you're not seeing your Taxpayer Registration Number (TRN) in your invoice template after saving it in the Account and Settings.

Entering or editing information in the Accounts and Settings will apply to future transactions. Hence already created transactions won't be updated.

For the TRN to show in your invoice, you'll want to manually add a Custom Field to it. Here's how:

1. Click + New button then choose Invoice under the Customers section.
2. Select Customize at the bottom of the invoice then pick the Edit current option.
3. Choose the Content tab then click the pencil icon ✏️ to edit.
4. Select the +Custom Field section.
5. Enter TRN in the box then tick it put a checkmark in the box.
6. Hit Done to confirm then go back to the invoice page. Fill out the needed details as needed such as the Customer name, date, etc.
7. Click Save once done.

You'll also want to print or send this invoice after creating it. You may refer to this article for the steps: How do I print or send an invoice in QuickBooks Online?

Moreover, I want to make sure you're able to customize your invoice in QuickBooks Online to personalize and capture the desired details  for your invoice: How do I customize my invoice in QuickBooks Online?

Please let me know if you hit any bumps along the way so I can assist you further. Always take care and have a nice day, @ffakhri.

Thanks Rey for the quick response, but this is not the solution Ideally when TRN number should be setup It should reflect in the Company info side below below customer details....

Im surprised even after many years for UAE Quickbooks failed to bring a reasonable template for UAE users..
Its just a linking of TRN Number and with the template...

Even UK Quickbooks always showed a VAT number on left above exactly where required. 

Hello @ffakhri,

Currently, the option to link the TRN number to the invoice template is unavailable. You'll want to follow the steps provided by my colleague @ReyJohn_D as a workaround.

I can send a product request to our engineers in your behalf. They can consider this feature added in the future update. I'll also share with you our page about managing customers and income in case you need some related articles.

Please know that you're always welcome to swing by anytime if you have questions. The Community is here for you. Cheers!