Why does Rules event 'After updating existing content' not result in 'Create a new comment'?
Part 1: Understanding the problem

It takes a little time/effort to get used (and become experienced) in using the Rules module.

My experience is that it is rare to find a bug in the Rules module. Most of the cases where I experience something in Rules that deviates from what I expected, I can find out what "my" problem is by using debug-facilities available for the Rules module.

So within the Rules settings, experiment a bit with the debugging facilities that come with the Rules module. At least have a look at the "Rules Evaluation Log" (to check that the Rules logic being executed, actually matches what you expect that logic to be).

Depending on what the result of that type of debugging is, you may also want to (temporary) enable the Devel module, so that you can (temporary) add a Rules Action like "Display a value".

For more suggestions about Rules debugging, refer to  "How can I display the value of a variable inside a condition?"

Part 2: Examples of creating comments after updating existing content

Sorry to say, but I disagree with this comment that was posted below this answer:


  It seems that it cannot work in this way and it is not possible to create a comment with this event.


Have a look at these answers, which are about questions that are variations of the question here:


How to create a Rule to create a new entity of type Comment and a body with tokens?
How to add a comment form in VBO to add the same comment to mutiple nodes?


Part 3: Experiment with other text formats

Even though your question doesn't provide any details about the text formats you are using, IMO it could well be that your actual issue is caused by the Text format that you're using for the comment body: your rule shows you're using filtered_html, but you should definitely give it a try to experiment with other text formats, such as plain_text.

PS: this answer is about D7 ...