A 1205 can be caused by air in the fuel system.
Make sure you have all the air bled out of the system.
To clear the code, set the switch to OFF, and press the ENTER key to clear the code.
If the code comes back, then you are going to have an internal or control issue that needs to be addressed.
Your unit is under warranty until Nov 19, 2020, and you will void your warranty by going inside the unit.
At that point, you will need to get an authorized Generac technician out there to repair this under warranty.
One of the possibilities is a bad controller, and this controller is very expensive. If you void your warranty by getting into the genset, you will be responsible for purchasing whatever parts it will need.