I was told to file a 1040x to add my granddaughter on it...I have custody of her and someone else has filed her previously

What are you trying to do?  If your only income is Social Security than you cannot get any child-related credits for claiming a child as a dependent.  Those credits come from having income earned from working.  

Were you trying to file and get a stimulus check?  If so, print sign and mail your tax return instead of e-filing it.  The IRS will sort out who could claim the child.  If someone else claimed her and should not have claimed her, they will face repayment of credits and penalties.

And do not send in a 1040X.  An amended return is only sent to correct a mistake on a return that has already been filed and fully processed.  It does not sound like you need to amend--unless we are missing some part of the story.  You only need to send in an original tax return to claim your grandchild, or use the non-filer site to claim her and get your stimulus checks.

I clicked on “amend” by mistake

https://ttlc.intuit.com/questions/2552802-how-do-i-cancel-or-undo-an-amended-return-in-turbotax-online