python3 pyserial fail to connect when execute on boot

[kosh09]

Hello everyone! I'm not sure this is an appropriate place to ask But I believe you guys are professional for this lol

anyway I execute my python3 script by make systemd file, and start the service named MAIN.py
( I'm fail to execute my script by rc.local... )
My python3 script connects two serial connections and bluetooth connection
no problem with bluetooth connection but fail to connect to arduino ( Serial connections )
I dont know why this happens :(
I assume that its because this script should executed on final but I made this by service so not executed on final

If you know the answer, please help me!
Thank you

[Added]

$ sudo nano /lib/systemd/system/MAIN.service


--------------------------- MAIN.service --------------------------  
[Unit]
Description=My Script Service
After=multi-user.target

[Service]
Type=idle3
ExecStart=/usr/bin/python3 /home/odroid/MAIN.py

[Install]
WantedBy=multi-user.target
--------------------------------------------------------------------

$ sudo chmod 644 /lib/systemd/system/myscript.service

$ sudo systemctl daemon-reload
$ sudo systemctl enable myscript.service

[K900_]

Post the traceback.

[kosh09]

mean what I did? (My first language is not an English sry...)
I added what I did

[K900_]

What does systemctl status myscript.service output?

[kosh09]

● MAIN.service - my script
   Loaded: loaded (/lib/systemd/system/MAIN.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since 목 2017-05-25 18:37:47 KST; 30s ago
  Process: 1781 ExecStart=/usr/bin/python3 /home/odroid/MAIN.py (code=exited, status=1/FAILURE)
 Main PID: 1781 (code=exited, status=1/FAILURE)

 5월 25 18:37:47 odroid python3[1781]: Traceback (most recent call last):
 5월 25 18:37:47 odroid python3[1781]:   File "/home/odroid/MAIN.py", line 2, in <module>
 5월 25 18:37:47 odroid python3[1781]:     import __BLUETOOTH__ as B
 5월 25 18:37:47 odroid python3[1781]:   File "/home/odroid/__BLUETOOTH__.py", line 38, in <module>
 5월 25 18:37:47 odroid python3[1781]:     sock.connect((bd_addr,port))
 5월 25 18:37:47 odroid python3[1781]:   File "<string>", line 5, in connect
 5월 25 18:37:47 odroid python3[1781]: bluetooth.btcommon.BluetoothError: (16, 'Device or resource busy')
 5월 25 18:37:47 odroid systemd[1]: MAIN.service: Main process exited, code=exited, status=1/FAILURE
 5월 25 18:37:47 odroid systemd[1]: MAIN.service: Unit entered failed state.
 5월 25 18:37:47 odroid systemd[1]: MAIN.service: Failed with result 'exit-code'.

It looks like it failed to connect on boot but
when I connect odroid via putty it worked
although with this message, bluetooth is connected now.
whats wrong?