It seems this is a server issue. GST website is a mistake.
You need to wait till the server issue is resolved. GST website is a result of bad programming, handled by infosys which seems to be incompetent.
Here are few steps you can follow:
- Clear all caches from your browser
- Clear all cookies
After this you should be able to login. If issue remains, contact GST tech team on twitter as service helpdesk is horrible and reply in several days.