Direct write redirection not work

What browser are you using? The plugin looks terrible.

Is https://zus-stochov.cz/ served by the same WordPress that Redirection is running on? What link works?

I have Chrome and Edge.

https://zus-stochov.cz/ is mounted on server as Alias and to https://zsstochov.cz/zakladni-umelecka-skola/ with this plugin: https://cs.www.ads-software.com/plugins/multiple-domain-mapping-on-single-site/.
https://prntscr.com/o4kek5

Default domain is zsstochov.cz, but Yes is on same WordPress.
When click on element, so it works (redirect correct from https://zus-stochov.cz/
to https://zsstochov.cz/zakladni-umelecka-skola/):
https://prntscr.com/o4irf5

The link on your page is for https://www.zus-stochov.cz/. This is being redirected to https://zsstochov.cz/zakladni-umelecka-skola/

The redirect you say is not working is https://zus-stochov.cz/ – note this does not have www. It appears you have only created a redirect for https://www.zus-stochov.cz and not for zus-stochov.cz. You will need to create one for both.