It stays off the serial bus until these three steps are done. Each one asks you a few questions and stores the answer in the device, which restarts for a moment. Do them in this order:

1. Button: which pin your page-break button is on (or that you have none). Set up…
2. Bus pins: which pins are wired to the C64's serial port. Needs the serial cable plugged in. Set up…
3. LED: which pin your status LED is on (or that you have none). Last, so the test skips the pins already in use. Set up…

Plug in the ESP32, click Connect, then print from the C64 to device 4. Pages show up here as the C64 prints.

Quick test: OPEN 4,4 : PRINT#4,"HELLO" : CLOSE 4