is MMDVM_Bridge executable?
root@dvswitch-server:/opt/MMDVM_Bridge# ls -la MMDVM_Bridge
-rwxr-xr-x 1 root root 3930476 May 23 19:33 MMDVM_Bridge

Is MMDVM_Bridge running?

root@dvswitch-server:/opt/MMDVM_Bridge# systemctl status mmdvm_bridge
● mmdvm_bridge.service - MMDVM_Bridge Service
   Loaded: loaded (/lib/systemd/system/mmdvm_bridge.service; enabled; vendor preset: enabled)
   Active: active (running) since Sat 2020-05-23 19:35:39 EDT; 53s ago
  Process: 12839 ExecStartPre=/bin/sh -c echo "Starting MMDVM_Bridge: [`date +%T.%3N`]" >> /var/log/netcheck (code=exited, status=0/SUCCESS)
 Main PID: 12841 (MMDVM_Bridge)
    Tasks: 3 (limit: 2200)
   Memory: 15.3M
   CGroup: /system.slice/mmdvm_bridge.service
           └─12841 /opt/MMDVM_Bridge/MMDVM_Bridge /opt/MMDVM_Bridge/MMDVM_Bridge.ini

May 23 19:35:39 dvswitch-server systemd[1]: Starting MMDVM_Bridge Service...
May 23 19:35:39 dvswitch-server systemd[1]: Started MMDVM_Bridge Service.

Or, look at it another way:
root@dvswitch-server:/opt/MMDVM_Bridge# ps ax | grep MMDVM_Bridge
12511 pts/1    S+     0:00 tail -f /var/log/mmdvm/MMDVM_Bridge-2020-05-23.log
12841 ?        Ssl    0:07 /opt/MMDVM_Bridge/MMDVM_Bridge /opt/MMDVM_Bridge/MMDVM_Bridge.ini

From MMDVM_Bridge.ini

General]
Display=Nextion

Nextion]
Port=/dev/ttyUSB0
Brightness=50
DisplayClock=1
UTC=0
IdleBrightness=20
ScreenLayout=0

From the log:

I: 2020-05-23 23:35:39.299 Display Parameters
I: 2020-05-23 23:35:39.299     Type: Nextion
I: 2020-05-23 23:35:39.299     Port: /dev/ttyUSB0
I: 2020-05-23 23:35:39.299     Brightness: 50
I: 2020-05-23 23:35:39.299     Clock Display: yes
I: 2020-05-23 23:35:39.299     Display UTC: no
I: 2020-05-23 23:35:39.299     Idle Brightness: 20
I: 2020-05-23 23:35:39.299     Screen Layout: G4KLX (Default)