Rudder-Server can't receive pending nodes : There is no readable input file at '/var/rudder/cfengine-community/inputs/failsafe.cf'
Before being able to update the policies you indeed need to get the first inventory on the server. The first step to check is if the inventory actually works locally on your node with:
# -i will give more information in case it doesn't work
rudder agent inventory -i
If the inventory creation works but upload fails (with a curl error) then you can check the apache logs on your Rudder server (in /var/log/rudder/apache2).