sol finally worked for me, thanks @karthikeyan5
i.e. wrong node version
$ /usr/bin/node -v
v8.17.0
$ /usr/local/bin/node -v
v12.19.0
so i changed my node in config/supervisor.conf
before
[program:frappe-bench-node-socketio]
command=/usr/local/bin/node /home/frappe/frappe-bench/apps/frappe/socketio.js
after
[program:frappe-bench-node-socketio]
command=/usr/bin/node /home/frappe/frappe-bench/apps/frappe/socketio.js