Is your catalog associated to another source?
I was having a similar issue and solved it about one hour ago. I couldn’t associate my FB page because it had a catalog created manually from the Business Manager.
To fix it, I followed these steps:
1. Access Business Manager
2. Click on Settings
3. Under Source Data, click on Catalogs
4. Select the existing catalog
5. Click on the button Remove at the top right corner of the Catalog box
However, I’m not sure this will fix your problem since it was an issue with my Facebook Page and I had to delete my Catalog.
But, that’s how I fixed my problem that perhaps is related to yours.