Stuck in installation

I used Kirby 2 for several years and never had a problem with its installation. Now I’m moving on to Kirby3
For some reason, I’m stuck right here:

My rootfolder


Could you give me a hint of what I might be doing wrong? Really appreciated.

If the folder cannot be created, this usually means that the folder/file permissions are not set correctly, and/or the files/folders are not owned by the server users (www-data).

Folders are 755, files are 644.
Aren’t those right?

It depends who is owner of the files/folders, because with these permissions only the owner is allowed to write. If the owner is the user who runs the webserver and/or the PHP process (usually www-data) it is ok, if not, the webserver/PHP can not write and create directories.