I havent created any github repo is it compulsory to make that? Also can we make changes in the game during voting period?

Leaving out the git repo probably just means you won't be featured on the git blog post. It's not going to stop anyone from playing and rating your work. As for changes - probably aren't meant to do anything now (except maybe bug fixes?) - but to be fair I uploaded all my screenshots after the submission time so ¯\_(ツ)_/¯