Converting a Desktop version to Online requires you to use the latest version of QuickBooks for Mac. If not yet updated, this might be the reason you're unable to export the .dmg file directly from QB 2014 to QuickBooks Online (QBO).
To help fix this, let's first download and install the free trial of the 2020 version. You don't need any codes to open it. Just click the QuickBooks Mac 2020 link through this article: Download a Trial of QuickBooks Desktop.
Second, you'll have to restore the .dmg file on Mac 2020. This transfers the data from your 2014 to the latest version. Here's how:
Double-click the .dmg file.
Drag the company file and the Attached Documents Library folder to the folder where you’ll store the company file.
Go to File from the top menu.
Select Open Company to restore it.
For detailed steps, see the Restore a .dmg file section through this article: Restore A Backup.