Welcome and thanks for posting in the Community, @nooraly-gmail-co.
Currently, QuickBooks Online (QBO) doesn't have an option to add serial numbers. However, you can use the SKU field to enter them as a workaround. This can be done by going to the Products and Services page.
To learn more about adding inventory products including SKU's in QBO, please check out this article: Add inventory products in QuickBooks Online.
Also, I recommend visiting our Blog site. This is where we share recent happenings and future developments, such as updates to newly added features. Here's the link: https://quickbooks.intuit.com/blog/.
Please let me know if you have any other questions or concerns by leaving a comment below. I'm always here to help. Have a good one!