Cannot get anything to setup on Home Assistant

I am new to Home Assistant but have not been able to set any thing up from DuckDNS to my NEST Thermostat. Everything I find and follow I end up with an error the latest is the NEST set up I cannot authorize the account get a 404 error. for such a so called easy program I have not been able to get anything to work and I have spent weeks getting it set up. Can some one help.

Are you using the official HA docs or some 4 year old youtube video which is long out of date?