That all looks good. I had no problems, but I’m using linux.
… Arduino Due support not properly installed had the same problem … this had the same problem in arduino-cli, and the suggestion was to remove and reinstall the boards (but no report if that fixed it).
I vaguely remember something: Using the arduino-ide, add the adafruit boards, and see if you can compile example:blink for the itsy-bitsy. How to add the boards: page 3 of https://cdn-learn.adafruit.com/downloads/pdf/adafruit-arduino-ide-setup.pdf