If your group has not yet reached the tipping point that you specified when you set it up, you cannot complete the group until more people join. If you want to complete it anyway, see this article.
If your group has been expired for more than two weeks, then you also won't be able to collect payments for it until you edit the deadline. You can tell if your group is expired by reading the text on big button on your dashboard. An expired purchase looks like this:
To fix the group, just click "Make Changes" and then move the deadline sometime into the future.