Hey Felipe,
Looks like you caught a bug. The code should work as is, but the text element’s text property is not accepting a value that is a number type. We’re on the bug and hopefully will have it fixed soon.
In the meantime, there is a simple workaround. You can convert the number to a string. Just replace the last line before the closing curly brace with this:
.then(json => $w("#textRate").text = json.rates[symbol].toString());