@hassio-jedi - Thomasloven recently helped me with this same challenge. The issue here is you cannot use the group domain with this card. In order to get this working you need to create a light group (someone else linked to the docs above my post). Here is what I added to my configuration.yaml in order to get slider-entity-row working.
light: - platform: group name: Great Room entities: - light.indoorkitchen_level - light.dining_room_level
What wasn’t immediately clear from the documentation page is the resulting entity name. In my case, the entity name is light.great_room. From there, you use the entity name for the new light group as the entity for slider-entity-row. This will allow you to change the slider value for all of your grouped lights at once.