@Pete_Skeggs See here.
If you don’t like the solution in that thread, you can fix this manually as follows:
sudo systemctl stop mozilla-iot-gateway
cd ~/.mozilla-iot/addons/zigbee-adapter
sudo rm -rf node_modules
npm ci --production
mkdir .git
sudo systemctl start mozilla-iot-gateway
You should note that you’ll continue running into issues while running the older Raspbian version, though.