Problem solved, but maybe someon has the same issue. I had to restart Steam on my PC, after that a symbol showed up beside the "install"-button in Steam on my Legion On S, where I could chose between installing and streaming.

The issue was:
I bought a Lenovo Legion Go S with SteamOS. When I go to games, that are installed on my PC, there should be a stream-button to play remote. but there is only an install-button. Steam is of course running on my pc while trying it. and steam on my legion go s shows tge pc in the remote play menu, so the pc is found. what can be the problem? thank you!