Using Occupancy Grids with ROS 2: frame problem and probability map

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I have been working on a Differential Drive project in Simulation with Gazebo. Everything is nearly set up, but it is giving me some trouble in the last stage (sending correctly the map). When I send the message of the occupancy map and try to visualize it in Rviz I encounter the following Error in the Transform section inside the Map::Status:

Could not transform from [] to [map_origin]

The frame map_origin is the one in which I put the map. In the OccupancyGrids docs, it appears no other frame that I could put in the message.

Also, in the ROS 2 version of the OccupancyGrid message, it appears to be that the value of the map is binary, although I see that it is optional. Does that mean that I could setup later the values I am putting in it?