Hard to tell from here.
1) RPi-Monitor always refuses to start if there exists only a root account on the system since it won't start a webserver as root (good). If this might apply to your setup then you have to add an unprivileged normal user first
2) Depending on the Debian version you're using the output of 'systemctl status rpimonitor' or 'service status rpimonitor' might help
3) In case you've 'lsof' installed then the following might help checking whether rpimonitor is started already or not:
- root@OrangePI:~# lsof -i | grep 8888
- rpimonito 465 orangepi 4u IPv4 9432 0t0 TCP *:8888 (LISTEN)
No lsof? Then a "sudo apt-get install lsof" would help
HTH