Aggregating data to use as legend for filled map

You can use OneDrive or similar to share sample file.

Try using the conditional formatting to change the colour of the filled map. You can do this based on field value using HEX colour codes within your dataset or with rules.

See step 5 here and come back with what specific questions you still have, as I imagine there may still be some: https://docs.microsoft.com/en-us/power-bi/visuals/power-bi-visualization-filled-maps-choropleths#create-a-filled-map