Hey, ​@ismoilahmad!

Check out this help article: https://help.manychat.com/hc/en-us/articles/14281308423452-Instagram-automation-troubleshooting

Some configurations on Meta's side can change permissions or other items wich “breaks” the integration with Manychat. Check if everything is as it should be.

You could also try to remove your IG account from Manychat and reconnect it again. But be aware that this action will delete all your Manychat Contacts. 

And, of course, you can open a ticket to the support team. They are able to investigate it further: How to get support from Manychat – Manychat Help

Hope this helps!