Hi Luke, it seems Meta is having some issues with this new feature and it is in beta version. So probably you can try again in few days. This is happening to some users.
However, try to Refresh Permissions as it helps to re connect your IG account. Usually this solve most of the issues.
You can do this by going to your ManyChat account and clicking Help → Refresh Permissions.
Here’s a step-by-step guide in case you need it:
https://help.manychat.com/hc/en-us/articles/14281308423452-Instagram-automation-troubleshooting
Hope this helps
Cata Rendon