Dreamweaver CSS 5.5 kann nicht installiert werden.

CS5.5 from 2010 is not certified to work on Win11.  It might work or it might not.  That's the risk you take.  You could try installing CS5 in  compatibility mode.  But I can't guarantee that will work either.  Computers have changed too much in 10+ years. 

Similarly, obsolete web authoring software has zero value today.  The web has changed too much.  

Best advice, get a modern code editor.  😉

CODE EDITORS:
-- Adobe Dreamweaver CC - https://www.adobe.com/products/dreamweaver.html
-- Codespaces (free, browser-based) - https://github.com/features/codespaces
-- Nova (Mac only, formerly called Coda) - https://nova.app/
-- Pinegrow - https://pinegrow.com/
-- Sublime Text - http://www.sublimetext.com/
-- Visual Studio Code (free) - https://code.visualstudio.com/
-- Wappler ~ Visual Web App Builder - https://wappler.io/

Hope that helps.