Multisite and Permalinks as %Postname%

I’m using a theme which suggest my permalinks as %postnane% but when I activate my multisite running subdirectories I’m not able to change my custom permalinks to: /%postname%/
I always get “blog/” before i can set my custom permalinks as %postname%
and cannot remove: blog/

My htaccess.file seems fine and wordpress tells me everything is ok…

Any suggestions would be appreciated . Thx in advance