When you're making a post and syndicating it to your Facebook business page, you may be asked by Facebook to reconnect to your personal page first, and then see a list of the business pages you manage. If you don't see the business page you want in the list of pages you manage, there are a few possible reasons and solutions:
The right Facebook Profile
The first thing to do is reconfirm that in the same browser, you are logged into the personal Facebook profile that is assigned as an admin or editor of the business page in question. If you are not, log out of Facebook in another tab of the same browser, and log in again to the correct Facebook personal profile.
Now leave the dashboard of your Business Control Panel for any other section, and return, and try again to make a post.
Meta Business Suite
If you've confirmed you're in the right Facebook profile and still don't see the page you manage, and you've confirmed that you are an admin or editor of the Business page in question, the problem might be Meta Business Suite. Some business pages managed in Meta Business Suite won't be permitted to use a third-party platform like ShopCity.com to post, and we haven't determined what the conditions are. The solution is to pull the management of that page by that Facebook profile out of Meta and re-establish the admin or editor role in Facebook Pages instead. There are three approaches:
For Pages with Multiple Admins/Editors: Remove the Person from Meta
1. In Meta Business Suite, go to Settings > Users > People to reveal the list of People with "Business Portfolio Access." Ensure that another person has full admin control, then click the three dots to the right of the person you want to post from ShopCity.com and click "Remove from the business portfolio."
2. Logged in as any full admin who hasn't been removed in Meta, go back to Facebook and switch into the business page using Pages. Choose Settings > Page Setup > Page Access and click Add New to send an invitation to the desired personal Facebook profile. When that person accepts, they will be an admin or editor but not through Meta.
3. Log out of your ShopCity.com Business Control Panel and log back in and make a post to check if the Business page appears in the list of Pages You Manage.
For Pages with Only One Admin: Remove the Page from Meta
Meta provides a full set of instructions for removing the page from Meta Business Suite here:
https://www.facebook.com/business/help/531142997352616?id=420299598837059
For Pages Where You Want All Page Managers to Remain in Meta Business Suite
The last alternative, if you don't want to remove the page from Meta or remove the operator from access to the Meta Business Suite, is to create another personal Facebook page exclusively for the purpose of managing the Business Pages that need third-party platform management. Have that page like and follow the Business pages concerned, and then have an existing admin invite the new personal page to have management access, making sure to send the invitation from Facebook Pages, not from Meta Business Suite. The user will have to be logged in to the new personal Facebook page in the same browser when posting from the ShopCity.com platform.