The LOGIN screen on Plann Desktop is from Facebook.
You’re logging directly into their database so we can access your 18 most recent images, analytics and other goodies.
Unfortunately, we have no control over them saying your accounts are not connected.
Firstly, please could you try following our troubleshooting tips on How to Set Up a Plann Desktop Account - Plann.
If you're sure that you've followed the steps, and are using the correct FB Business page login, there are a few things that could be happening:
Do you have a Facebook Business Page linked to a Facebook Business Manager account?
It's a very recent change, but this may be why your Facebook page is not showing up during the authentication process.
Here is a link that explains How to add a Facebook Business Page to Business Manager.
Have you linked multiple Instagram accounts to the FB business page?
You may need to create a new FB business page for the Instagram Business account you want to login to on Desktop.
You may not have granted Plann all permissions to manage your page/s?
If so, please check the privacy settings through your FB Business page settings.
Try switching your Instagram account to personal then back to business again and reconnect the FB Business page to refresh.
If you're still having trouble, please contact Facebook directly for Business page troubleshooting.