Hello there, @Jimrudin.
I believe you're referring to your customer's email address that should be converted from QuickBooks Desktop to QuickBooks Online after migrating your data.
Since the two products have different features, the system doesn't transfer all your customer's information. Still, an E-mail is one of the details that can be transferred. Please see the attached lists below.
To learn more about what features move and don't move to QuickBooks Online, you can check this article: Find out how your books will move from QuickBooks Desktop to QuickBooks Online.
If the information isn't showing in your QBO account (even if listed as capable of being converted), you'll want to access your QBO account in an incognito window or different supported browsers.
Also, you'll want to get answers to frequently asked questions about converting your account: Learn about converting from QuickBooks Desktop to QuickBooks Online
Please let us know if you need further help. We're always here to assist you. Take care and have a good one, Jimrudin