It’s possible they installed with sudo or something, which ruined the permissions. First try find /home/lewis/.steam ! -user lewis. That will show if any files got owned incorrectly. If so, do chown -R lewis:lewis /home/lewis/.steam.
Not sure this is a permission/owner issue though. My guess is /home/lewis/.steam/debian-installation/steamapps/common/SteamLinuxRuntime_sniper/_v2-entry-point: 285: exec: /home/lewis/.steam/debian-installation/steamapps/common/SteamLinuxRuntime_sniper/run doesn’t have the executable bit. try chmod +x /home/lewis/.steam/debian-installation/steamapps/common/SteamLinuxRuntime_sniper/_v2-entry-point: 285: exec: /home/lewis/.steam/debian-installation/steamapps/common/SteamLinuxRuntime_sniper/run.