In most cases, this is because you did not purchase the Signature in Step 3 using the drop down menu.
If you wish to do this - when you create your label you must select the RETURN RECEIPT (SIGNATURE) option in the drop down on STEP 3 of the Single Label. This will add the additional $1.35 USPS Postage to purchase the Signature.
You must purchase the signature at the time of mailing to receive the PDF report of the Signature. It is returned back to your account and can be opened using the Return Receipt Signature Link.
Laws are changing in the US and today many laws do not require you show who signed for the Certified Mail letter they simply require you have proof that you mailed the notice "Certified" and you must provide "proof of delivery". USPS Certified Mail proof of Delivery is the date and time stamp recorded by the United States Postal Service. This electronic date and time stamp is authored by USPS serves as evidence of letter delivery.