register_activation_hook not creating table

You create auto column sno int(10) NOT NULL AUTO_INCREMENT, and forget to define it as key, add PRIMARY KEY (sno) to your sql.

$sql = "CREATE TABLE $table_name (
      sno int(10) NOT NULL AUTO_INCREMENT,
      postid int(50) NOT NULL,
      subpostid varchar(5) NOT NULL,
      votingcount varchar(50) NOT NULL,
      postclicktime timestamp(5) NOT NULL DEFAULT CURRENT_TIMESTAMP,
      PRIMARY KEY (sno)
    )  $charset_collate;";

If still won’t work, try replace postclicktime timestamp(5) with postclicktime timestamp.
Before MySQL 5.6, MySQL did not support fractional seconds stored as part of a TIMESTAMP datatype.