ERROR: Material doesn't have a color property '_emission'

This is because the material you want to modify does not have a property named _emission. Maybe what you are looking for is _EmissionColor?

To find all properties for a materials shader do the following

1. Select the material you want to modify

2. Select the gear from the top right corner of the material

3. From the dropdown list select Edit Shader...

4. The inspector will now show all the property names for the materials shader that you want to modify

See the attached image for an example