We're unable to convert a company file from an old or unsupported version of QuickBooks to QBO. You'll need to move your file temporarily to a trial version of 2020 where you can migrate your file. Here's how:
Thanks for sending. I followed the steps but the restored file, nor the original file recognize the username/password we've been using to login, when trying to open in the 2020 desktop version. Any suggestions? thanks!
Thanks for keeping me updated about this, @atabak.
In this case, I recommend resetting your login credentials using the Automated password tool for QuickBooks Desktop. You may access your QuickBooks Desktop 2015 file and modify the credentials from there. This way, you'll be able to have new login information and start creating the backup and restore it on the 2020 version.
Once done, you may now start restoring the back up then migrate it the Online version. I've attached here the steps that you're going to follow so you can accomplish these processes: