The Community has you covered, @sanjujeev2013-gm.
At this time, the unit of measure functionality is only available in QuickBooks Desktop. As a workaround, you can add the UOM info on the Mesage on invoice box of the invoice.
As another option, you can utilize an integrated third-party app in QuickBooks Online. QBO does offer the support of thousands of various third-party applications for any functionality you may need. To look for an app that can help you accomplish this, head to the Intuit App center and type a keyword like "Inventory management" or "Unit of measure" in the search-bar.
You can follow these steps:
Refer to these articles for more info:
For future reference, the following articles outline the steps on how to personalize the look on your forms:
Lastly, I'd like to let you know that our developers are constantly uncovering and considering new ways to improve the program. To be updated with our latest news and enhancements, you can visit our QuickBooks Online Blog.
Stay in touch if you have any other questions or concerns. I’ll be right here to answer them for you. Enjoy the rest of the day.