Hi, you need to press the underlined marked text: "Make compositor fullscreen" a few times directly after the message appears.
If this does not resolve your issue try to restart the entire hmd and / or go in steamvr settings and hit the button "direct mode" ( again ).
For me I need to press the "make compositor fullscreen" marked text everytime after starting steamvr beta...It seems like it is a gimmick from valve :D