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What I do wrong?

Nothing really, other than not checking whether rosbridge_suite is actually released into ROS Melodic.

As you can see both on the wiki page that you link us to (note how the version selector right under the package name at the top of the page doesn't have a button for melodic), as well as on status_page/ros_melodic_default.html?q=rosbridge, the package(s) simply hasn't (haven't) been released yet for Melodic.

So apt telling you it can't find the package is actually correct.