can I remove or deactivate the function "Focus". On the train it annoyingly prevents to receive messages.

If Focus is enabled, you will not receive notifications about messages and calls. To deactivate Focus drag your control center down from the upper left corner of your display, find Focus in the menu and deactivate.

If you want to prevent your contacts from being notified when Focus is activated, then you can deactivate that function here: Settings -> Messages -> Disable in Focus at the top of the menu.

You can read more about Focus here:

Use Focus on your iPhone, iPad, or iPod touch - Apple Support