It should be in JIRA_HOME/log
If you are using Scriptrunner there is the history of the last 15 execution that display the log by clicking on the faild or success logo.
If you are not able to see it in the file, you need to change the log level or log using log.error()
JIRA_HOME is on the server and the path that have been defined during the installation.
If you want to set the log level within the script you can do it like that:
def log = Logger.getLogger("com.test.listener")
You can also check the log from the JIRA using Scriptrunner by going to Built-in Script in Scriptrunner > view logs and select atlassian-jira.log that is the file you are looking for .