I'll walk you through the steps in converting QuickBooks Desktop to Online.
Since you're not getting the option to export, you need to reset the update of the the program first. Updating the software fixes the application error.
Also, make sure you're using the supported QuickBooks Desktop version (Quickbooks 2017-2019).
Once you're done, you need to reboot your computer. Then, try exporting again.
Here are the steps:
Please refer to this article for more information: Convert QuickBooks Desktop file to QuickBooks Online.
I'll be around if you have additional questions. Feel free to post any time.