Systemd service unit failed with result 'exit-code'

Question

I have a python script that works fine if I run it with /usr/bin/python3.

But I would like to be able to run it from a bash file as root through a service unit from systemd. I want to be able to pass a bash script because I would like to chain this python script and then a php script afterwards).

For this, I have created the following service (without timer for the moment):

[Unit]
Description=My service

[Service]
User=root
ExecStart=/usr/bin/bash /home/myself/script.sh
Type=oneshot

and a simple bash script :

#!/bin/bash

python3 /home/myself/script.py

but when I run "sudo systemcl start myservice.service", I have this error :

Job for myservice.service failed because the control process exited with error code.
See "systemctl status myservice.service" and "journalctl -xe" for details.

and when I look at the status :

..... systemd[1]: Starting My service...
..... bash[13176]: Traceback (most recent call last):
..... bash[13176]:   File "/home/myself/script.py", line 33, in <module>
..... bash[13176]:     files_number = next(os.walk(files_path))[2]
..... bash[13176]: StopIteration
..... systemd[1]: myservice.service: Main process exited, code=exited, status=1/FAILURE
..... systemd[1]: myservice.service: Failed with result 'exit-code'.
..... systemd[1]: Failed to start My service.

I don't understand why it doesn't work. Is it because I'm using the 'os' library in my script or something else I don't understand?