how are you!
First you must know that in order to do this you must be the owner of the fanpage and ig account to refresh the permissions when unlinked, if not you must ask the owner to do this from their manychat account
If you are the owner, send a ticket to support so that they can send you the step by step to remove the permissions from your Facebook page and re-enable it, it already happened to me and that solves everything.
I hope I've helped

I don't understand what you mean. My account is now the owner. So I have to take steps to remove permissions from the Facebook page and reactivate them?

Exactly, you must do that if you are an owner, you must remove all manychat permissions, save them and activate them again. then you go back to manychat and clicking on refreshing the permissions will solve it

do this:

Please switch to your personal Facebook profile, go to your Business integrations https://www.facebook.com/settings?tab=business_tools, click View and Edit next to Manychat, and toggle off all the options:

Then go to Manychat and refresh permissions through the Help button - it will take you here:

Click on Choose what you allow and make sure all the permissions are granted both for the IG account in question and the FB page that is linked to it. Then, click Continue and see if it helps.

You can also check this screencast for reference:

You go into the link you told me, turn off all permissions, and then come into maychat to grant permission again, and the message "You have influential rights to reload the account" still appears at the top.

Should I delete the linked Instagram account and try again?

Send a ticket to support then because that should work perfectly.
Sorry for not being able to help you solve it.

don't do it until support tells you to