Probably, you or an opt-out automation unsubscribed this contact earlier in ManyChat.

I have the same problem. Did you find a fix?

Hello ​@GraceS ,
Is this happening with all contacts or just one in particular?
This usually happens when a contact has been unsubscribed.
If you're using a free account, please check whether you've exceeded the limit of active contacts allowed for your plan.

It is happening with 18 contacts since updating to the PRO paid plan. I just refreshed the connection and after that it seems to make the new contacts active, but the old contacts havent received the automation, since the first action was adding a segment.. and when that does not work, the automation stops.. sadly. 

If you're unable to update information such as adding a TAG or modifying a custom field for these users, it's possible their subscription was canceled — either due to reaching the contact limit on the free plan or because it was manually canceled.
Check the contact's profile to see if the message "Unsubscribed" appears.

If that's the case, depending on the channel, you may be able to delete the contact and add them again. For channels like WhatsApp and SMS, for example, it's possible to register the contact manually.

Again, I have a paid plan. And how can they unsubscribe? Especially when they did not even got the second message?

Hello ​@GraceS ,
When a user contacts us on Instagram, they remain active for an indefinite period — this is the default behavior.
If the user's subscription was canceled, it required an action, such as the user blocking you, or you canceling it manually or through an automation.
There isn’t a specific field that shows the exact reason for the cancellation. If you would like to investigate further, it might be worth submitting a request to the support team:

https://help.manychat.com/hc/en-us/p/get_support_page

I throughly think it is/was a bug. Since they were from insta and it only happened with the step to add a tag. Then they got unsubscribed or something. They did not block me, since some reached out for the link they did not get through many chat.