As of December 1, it is not possible to log in via Blizzard through bungie.net, so you cannot unlink that account or set up Cross Save while it's linked. Please fill out the contact form at the bottom of the following article and tell Bungie that you would like to enable Cross Save but your Battlenet account is preventing you from doing so.
https://www.bungie.net/en/Help/Troubleshoot?oid=48099
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I will do just that, thanks for the help! ^-^