I've assigned page for 404 error handling (example.com/404).
I've created Rules condition: Data to compare: [site:current-page:url], Data value: 404.
And I've created some Rules action for this case.

But when I open nonexistent page (example.com/wrong) - there is no corresponding Rules action.
It is because I see 'example.com/wrong' address instead of 'example.com/404'

But how can I create Rules condition for event when 404 error generated?

I set a page for the 404 error. I created the Data to compare: [site:current-page:url], Data value: 404 condition and an action of that condition.

When a non existing page is requested, no action is triggered. I guess it happens because the page URL is https://example.com/wrong, instead of https://example.com/404.

How can I create condition for a 404 error?