That is because site is down. GST server is down most of the time. Wait for some time and try once server is free.
As per my knowledge as of now, server can handle 50,000 users at a time which is very less and many people facing same issue.
The error you gave comes when SSL certificate is not valid.
Check you computer date and time. Sometime when computer date and time are not correct you may get this error.
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