How long before I can amend a 2017 return

I filed my 2017 returns two weeks ago and my return was accepted.  As I owed money, I noticed the money was taken from my bank account last week.  Problem is that I just discovered that I forgot to enter a stock sale from December so I need to amend my return ASAP and pay more.

 

When I go on the TurboTax website to try and file a 1040x, my 2017 filing does not appear.  Shouldn’t it be there already?  Since I owe money, I’m anxious to make the correction and avoid a fine.  How long should it take before my 2017 return is available on TurboTax.com?