I made a script of html and then it had to make a Signin form and a signup form which it did but after signing up the browser should go to the same page but show a message that "your account has been made! Signin to continue" but it shows the error 405 not allowed. Same with the signup form but this time it should send me to the home page but instead it gives the error. So, is this because of github pages?
405 is the HTTP status code for "Method Not Allowed".
Sign up forms generally make POST requests to be serviced by a server side program.
Github Pages does not support server side programs, which means it does not make sense to make a POST request to a URL hosted there. Consequently, you will get a Method Not Allowed error if you try to make a POST request to a Github Pages URL.