Linear color space is not supported.
Luna targets WebGL 1 API, which often lacks the appropriate extension to support the reading of texture data in a linear fashion. Due to that limitation, it is impossible to support rendering in Linear space.
Steps to fix
Switch the color space used by the player to "Gamma". This can be done by navigating to
Edit > Project Settings > Player in main Unity menu:
You can use a separate build target (i.e. WebGL) to amend the color space setting to keep original project untouched, if switching to Gamma space globally is undesirable. In order to do this, please switch to WebGL build target before running the export.