Sonoff Zigbee 3.0 USB + Zigbee2MQTT

Hi guys, disclaimer: I’m new here and know nothing about scripting or coding.
I’ve bought my RPi 4, the Sonoff Zugbee 3 stick, and some IKEA switches because I’ve read that you can use them this way (much cheaper). I’ve installed HA, added the zigbee stick, and want to add my switch via Zigbee2MQTT (not ZHA, because I’ve read that it doesn’t see the 1743 switch only its battery - tried it, confirmed it’s no use).

My problem is, after installing Zigbee2mqtt and the Mosquito thingy that I’ve read I also need, it just doesn’t start, and I can’t reach the web UI (Bad Gateway 502).

I’ve read the other posts but their error l;ogs didn’t look like mine so I haven’t started following instructions I’ve read there (they referred mostly to port numbers etc, but that doesn’t semm relevant in my case).

Where do I begin? All help is greatly appreciated.

My zigbee2mqtt log is saying:

> [email protected] start
> node index.js
Zigbee2MQTT:error 2021-12-17 23:19:53: Error while starting zigbee-herdsman
Zigbee2MQTT:error 2021-12-17 23:19:53: Failed to start zigbee
Zigbee2MQTT:error 2021-12-17 23:19:53: Check https://www.zigbee2mqtt.io/guide/installation/20_zigbee2mqtt-fails-to-start.html for possible solutions
Zigbee2MQTT:error 2021-12-17 23:19:53: Exiting...
Zigbee2MQTT:error 2021-12-17 23:19:53: Error: Error while opening serialport 'Error: Error: No such file or directory, cannot open /dev/ttyACM0'
    at SerialPort.<anonymous> (/app/node_modules/zigbee-herdsman/src/adapter/z-stack/znp/znp.ts:146:28)
    at SerialPort._error (/app/node_modules/zigbee-herdsman/node_modules/@serialport/stream/lib/index.js:198:14)
    at /app/node_modules/zigbee-herdsman/node_modules/@serialport/stream/lib/index.js:242:12
[23:19:53] INFO: Handing over control to Zigbee2mqtt Core ...

Maybe show your Zigbee2mqtt configuration ?