Code 1505 is a Cranking Fault.
This can be caused by a few different things.
Your unit is still under warranty. If you attempt to work on this yourself, you will void your warranty, and you will be responsible for the cost of any parts.
My recommendation is that you contact an authorized Generac technician to have this repaired under warranty.
If you need help locating one, I can help you if you provide me with your ZIP code.
If you do not mind losing your warranty, and buying your own parts, I can walk you through the troubleshooting.
Be advised, though, that one of the possibilities is a bad controller, and this controller is very expensive.
Another possibility is a bad coil. Although not an extremely expensive part, it does require extensive disassembly of the genset to change.
Let me know how you wish to proceed.