RFID module RC522 interfaced to Pico results in various Uncaught Error MFRC522 Request Errors

In:

var err = this.r(R.ERROR);
if (err) throw new Error("MFRC522 Request Error "+err);

It's reading the ERROR register, and that is reporting the number 255 - so you'd have to look at the datasheet itself for reasons it's so unhappy.

You could try checking inside getCardSerial - if it doesn't receive the correct bytes it just returns [], so you could print out what it did receive and see if that makes sense