How to upload a map on Leaftlet? Error in addPolygons and object cannot be found

An unused argument error most often means that one made a typo while spelling the argument.
In your case you spelled fillcolor where the function expects fillColor.

In general it is good practice to make a small (as small a possible) example that contains all the elements that play a role in your problem. See FAQ: What's a reproducible example (`reprex`) and how do I do one? .
In this case I expected the typo (from my own experience) and supposed you were using the leaflet package, so the answer of your question was easily found. But otherwise I would not have known where to find districts, pal and fb.