“Sign Documents” API only programmatically signed signatures for Sender of the package.
In order to sign documents for a Signer (signer with id of “sender” in your case), you’d follow this guide “Bulk Sign For a Signer” in three steps:
#1. generate an authentication token for signer
#2. generate a session token out of this authentication token
#3. call the same API as you did, but put session token in cookie header as credential in stead of api key.