There may be a possibility that you are following over 7,500 accounts.
For most accounts, the maximum number of accounts that you can follow is 7,500.
If you are following 7,500 accounts, you will not be able to follow anymore accounts automatically
and receive an email from Instagram stating that you cannot follow any more accounts.
If that is the case, you will need to unfollow some accounts before you can use the auto Follow function again.