For the correct work, the action needs to receive document ID by one of the following ways:

1. From the Company Drive - the easiest way.
In this case the parameter  {=Document:ID} is used for "Source"

In this case the field needs to be added. Type of the field - File

The parameter of "Source" - {=Document:PROPERTY_FILE}

3. Any other document.
Any document from the Company Drive can also be used.
In this case, you need to transfer ID of the element of the drive to the parameter "Source".
For example, in case of the endorsement of the document by the second user, you can use the link for the document, which was  created in the first stage of the approval process. In this case the value of the parameter "Source" will be similar to {=A69709_72962_84139_92897:diskId}.

In this example A69709_72962_84139_92897 - is the ID of the action, which endorsed the document in the first stage of the approval process.