How are you? You must authorize the tool from your personal Facebook account and click on the account that you are not able to connect. You do this from:
>> click on profile photo (top right)
>> Settings and privacy
>> Settings
>> among all the options in the left column, look for: >> your activity >> business integration
There, in assets, click on "edit" in manychat and click on all the options and verify that the IG account you want is selected. If not, select it everywhere
>> go back to manychat, refresh accounts and it will appear

Good luck and greetings

Pablo

(This was created with google translator)

I followed all the instructions, but it still doesn't work.