Facebook now has a separate "management" page, called the Facebook/Meta Business Suite.
Page access given here is different than page access given through Facebook itself, and does not immediately allow access to stream through StreamYard.
How to provide someone streaming access to a page through the FB/Meta business account:
You must have your Facebook Business account tied to your personal Facebook account that you sign in to facebook.com with.
- Invite yourself to your Facebook Business account.
From the People page, for the Business Account, click Add people.
They do not need to be given "Admin access".
- Assign the Facebook page. Click on the user, then click Assign assets.
- Select the page(s) to assign to them, and enable Publish content access.
Then, click Save.
- You should now be able to connect the Facebook page on StreamYard normally.
Make sure you've signed in to facebook.com as the Facebook account that was provided access.