I migrated my data from the desktop version to the online version, where can I find the transaction number

Welcome to the Community, @athenasit.
 

Before diving into finding the solution to your issue, may I ask what kind of transaction number you were looking for when you migrated your data from QuickBooks Desktop to QuickBooks Online?
 

I’ll be waiting for your response. If you have more details to add, kindly add them to your reply. Have a good day!