[A] If you are unable to edit the draft document, open the document and check if there is a "Cancelled" label attached on upper left side or not.
  

[B] If there is cancelled label on the document, that means you have cancelled the draft document which can not be edited.

[C] In this case you will have create new document again.

[D] If you can not find cancelled label and still you are not able to edit the draft document then contact our live chat support.

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