Welcome to the Community Forum, G.W. Suresh. Currently, QuickBooks Online doesn't have a tax invoice template that follows the format mandated by the Sri Lankan government.
To have this feature added in QuickBooks specific to your country, I recommend sending feedback. Here's how:
Go to the Gear icon and select Feedback.
Enter your feedback in the description box.
Hit Next to send.
In the meantime, I suggest using a third-party app that provides the required template.
Feel free to post again if you have any other questions.
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