I get this warning when the passwords don't match. I can't find any selecotr in DOM?

You can always use

cy.contains

to match on the entire text if you want to validate the text appears. If I recall certain toast messages are really hard to get selectors for and it would depend on what framework you are using. https://docs.cypress.io/api/commands/contains

I've already tried with cy.contains and it can't find this text.

Do you have any code snippets you can share of what you've tried?

that popup is not in the DOM, that is browser showing you outside the DOM, See https://glebbahmutov.com/cypress-examples/recipes/form-validation.html

You are a life saver. And a great teacher. Keep up the good job and keep inspiring people. Thanks a lot!