Welcome to the forum @oswald
Are you sure you are using a JS query and not a JS transformer? The rules on how you reference Retool's state are different with each. (Tip for writing transformers - Retool scope - #3 by victoria, Transform data with JavaScript)
If your REST query is named exactly BTCZAR then the js query should be seeing it. Also check with the State viewer to make sure .data.lastTradedPrice actually exists prior to triggering your query, it may not actually be there or have a different spelling.