Hi Vishal,
Past January 30th of this year, second generation Cloud SQL has replaced the first generation, and past that date, first generation Instances will no longer be able to be created, which is why you cannot `Clone` the Instance.
You could upgrade to the second generation, if that is something that will work for you, by following this documentation[1]. Cloning Instances is now only available for second generation Instances.
[1]
http://cloud/sql/docs/mysql/upgrade-2nd-gen