I am reading about the error 504 Gateaway Time-out error. All the websites I have found, relate the 504 error to HTTP status code. the error is displayed if visiting a https website. According to what I learned in a computer security curse https is more secure than http. However I have only found HTTP status codes. Why I cannot find the https ones?
Because they are exactly the same.
HTTPS is HTTP with added encryption, but the actual application protocol doesn't change; the requests and responses between clients and servers use the same syntax and status codes.
You can think of it as speaking English on a secure phone line vs. an insecure one; regardless of the line security, you'll still be speaking English.