It's nice to see you here, @jessi3,
If you want to convert a QuickBooks Mac 2015 file to QuickBooks Online, you must update the company file to the supported version of the software.
This means that you need to restore it to a file that is 3 years higher. This means that you will need to restore the backup to a Mac 2019 file, create another backup copy there, and then restore it to 2021 (2015 > 2018 > 2021).
To get help extract data from the 2015 file and assist you with the upgrade, tap this link to start a chat with a live rep: QuickBooks Desktop for Mac chat
You can read through these articles for more information about the conversion:
Kindly post here again if you have any additional questions about the upgrade. I'll be more than happy to assist you. Have a nice day!