Unable to automate scan with Nessus 7 professional

I am evaluating the product Nessus 7 to perform vulnerability scans on the systems in my network.I am able to perform the scans successfully, but I am unable to automate it with a python Nessrest client. The following error is thrown by the nessrest client. Please help me out with this issue. Thank you.

 No Title
 *****************START ERROR*****************
 JSON    :
 {"uuid": "ab4bacd2-05f6-425c-9d79-3ba3940ad1c24e51e1f403febe40", 
 "settings": {"text_targets": "targetIp", "file_targets": "", 
 "folder_id": 82, "description": "Created with REST API", "filters": [], 
 "launch": "ON_DEMAND", "scanner_id": "1", "emails": "", "filter_type": "", 
 "name": ""}}
 {}
 HEADERS :
 {'X-Cookie': 'token=1c1c4f11e325eb19440feaaf04706d1265f347d105a06f44', 
 'Content-type': 'application/json', 'Accept': 'text/plain'}
 URL     : https://ipaddress:8834/scans 
 METHOD  : POST
 RESPONSE: 412


 {
   "error": "API is not available"
 }


 ******************END ERROR******************
 Traceback (most recent call last):
 File "nessrest.py", line 1, in <module>
      import nessrest
 File "C:\Users\ballalc\nessrest.py", line 8, in <module>
      scan.scan_add(targets="ipaddress")
 File "C:\Users\ballalc\ness6rest.py", line 705, in scan_add
      self.scan_uuid = self.res["scan"]["uuid"]
 KeyError: 'scan'