We are sorry for the trouble. As I have understood, you are trying to add the signature field to the document which is already signed by your client.
Once the PDF is signed and the signature process is complete the PDF becomes read only and gets locked for further changes. We can not edit or sign the PDF once it is already signed.
You need to get a new copy (Unsigned) of the PDF file and then while sending the file for signature you need to add your client and yourself in the recipient list in the order you want the signatures to be made, so that once the file is signed by the first person it will come to the second one for signature. Once both the parties signs the PDF it will then become read only.