You need to check for !is[system] BEFORE you get the created field value; otherwise, you are checking to see if the created date value is a system tiddler! You can omit has[created] since get[created] will only return a value if the field exists and is not empty. Also, you can eliminate one join[_].
Try this:
[!is[system]get[created]] [get[title]lowercase[]split[ ]] +[join[_]]