If I have filed my taxes can I go back an get advance

Sorry-   if you already e-filed you cannot go back and get the advance.   The advance had to be offered to you before you filed, so perhaps you were not eligible.

No.  If you already filed it is too late.  They have to offer it to you when you file.   You can't self select it. Then you don't get it until AFTER your efile is Accepted by the IRS which won’t be until Jan 27 or29.  
https://turbotax.intuit.com/refund-advance/