Did you go all the way to the end, though the 3 steps and hit the big Transmit Returns Now button? If you efiled you can not cancel it. Now you have to wait until Jan 26 when the IRS opens and they either Accept or Reject it. If they reject it you can fix it or leave. But if they accept it, you have to wait until the IRS fully processes it and sends the refund or you have to pay any tax due. THEN you can amend it to change it to be right.
When you efile you get back 2 emails. The first email only confirms the transmission. The second email says if the IRS (or state) Accepted or Rejected your efile.
When you log into your account you should also see the status and if it was Accepted or Rejected, Started, Printed, Ready to Mail, etc. What does it say?
Why do you need to cancel?