Hi Haddnin,
I am not sure of the element exists condition, but I used a different way to accomplish similar request.
Store the error message value in a data element. the error message will be available either in innerHTML or innertext or value.
Now use this data element in the Event based rule using 'data element changed' condition and now the rule will get triggered whenever the text appears or changes.
you can even pass the error message to a prop to understand what message was displayed to the user.
Let me know if this works for you.