I've got your back, Imri.
Are all of your invoices didn't convert? Or there are some invoices were not? Some invoice features in QuickBooks Desktop (QBDT) aren't available in QuickBooks Online (QBO), just like progress invoices, this won't convert to QBO. You'll want to manually enter them into your QBO account.
You might want to check out this article to learn what data doesn't convert when migrating from QBDT to QBO: What to expect when you switch from QuickBooks Desktop to QuickBooks Online.
Also, take time to read this article to know how to get started with QuickBooks Online: Getting Started With QuickBooks Online.
Keep in touch if you need any more assistance with this, or there's something else I can do for you. I've got your back. Have a good day.