I have a Visualforce page and a controller extension class. I know my VF page is invoking the extension class but why my system.debug('blah blah blah') in my controller extension class is not showing anywhere?
I created a New Trace Flag but still I don't see the system.debug message in my debug log or developer console log when I run the code.
Update
When I open Dubug->Change Log Level.. -> I then got this error message I am not sure what it means
When using a apex:commandButton with an action= attribute, SFDC will first decode the view state, then use the values from the form (or enclosing actionRegion) by calling the respective setters in the controller and then invoke the action method.
If the setters blow up with an uncaught exception, the action method will never get invoked and your debug statement will never appear.
This is all documented in the VF Order of Execution (excerpted below - you may be failing at step 2)
If you are logged as diffrent user, you have to enable debug log tracking for that user. Go to Setup -> Logs -> Debug Logs, then new and select user that you would like to see debug log. More informations here: https://help.salesforce.com/articleView?id=code_add_users_debug_log.htm&type=5
Also in developer console you have to uncheck Debug->Show my current logs only.
Best Answer
This is one confusing area of Debug Log: you always need to set it on a User because class/trigger debug flags don't actually generate logs.
See documentation