Hello,
If your database version is earlier than 5.6, then CONVERT_TZ() will not work and you can use DATE_SUB() or some other method. I believe you should remove some extra characters from your query, so rewrite your code like so:
$sql3 = "SELECT COUNT(*) FROM $walletTable WHERE DATE_SUB(created_time, INTERVAL 6 HOUR) >='$fromDate' AND DATE_SUB(created_time, INTERVAL 6 HOUR)<='$endDate' AND account='$account' AND category='move' ";
If you don’t trust your inputs, you should use $wpdb->prepare() instead of relying on PHP string interpolation to insert the variable values. But if you are in control the inputs it should not matter.
with regards