Does it work when set to run on a time interval, rather than on a schedule?

How many clusters is this running against?

Setting it to run on a time interval is something I tried as well without success.

if you want, you can add Debug code as follows.  You can find the log entries in the most recent management log.

derivedMetricRulette = "derivedMetric.script.ClusterGrowthRate @ VMWCluster = " + scope?.name    // if you get an error, shows the source and scope better in the management logs

org.apache.commons.logging.Log log = org.apache.commons.logging.LogFactory.getLog(derivedMetricRulette);

if (!scope) {

     log.error("scope is null")

     return null

}

ds = server.get("DataService") // Data Service

valueThen = null
numdays = 180

while (valueThen == null && numdays > 0) {

    try {
        String query = "virtualMachinesPoweredOnCount from VMWCluster:name = '$scope.name' for 1 day $numdays days ago"
        valueThen = min(server.QueryService.queryObservations(query))
    } catch (e) {
        // log.info("valueThen = min(server.QueryService.queryObservations($query)) : " + e.toString())       // comment out this line once you figure out what's wrong, will create a lot of log entries
    }
    //if (valueThen >= 0){ // code not needed
    //    break
    //}
    numdays--
}

try {
    valueNow = ds.retrieveLatestValue(scope, "virtualMachinesPoweredOnCount")?.getValue()?.max // Retrieves the latest available value, not necessarily current

    if (valueNow && valueThen && numdays > 0) {
        growthRate = Math.ceil(((valueNow - valueThen) / numdays * 30)) // Calculate current growth rate
        return growthRate
    }

} catch (e) {
    log.error("valueNow = ds.retrieveLatestValue : " + e.toString())
}
return null

I only have 1 cluster in this environment

Do you have it on entry or exit of the schedule?  Not that it should matter, but with both turned on it seems to generate for me.