I can't run Traccar as unprivileged non-root user

Hello people,
finally my Traccar installation is working very good.
Now, I've decided to secure it and, following the documentation, I'm trying to Traccar run as non-root.
So, logged on my shell, I did:

sudo adduser traccar

I add my nice password, then

sudo mkdir /etc/systemd/system/traccar.service.d/
sudo touch /etc/systemd/system/traccar.service.d/run-as-user.conf

then I've added

 [ Service]
 User =traccar
 Group =traccar

into 'run-as-user.conf file'.

Now, because I'm a beginner with Linux,
I don't know how make /opt/trraccar to be readable and writable by 'traccar' user.

Current owner group is 'root':

drwxr-xr-x 10 root root 4096 mar 13 23:57 traccar

Someone can help me, please?
What commands I have to give to my shell to secure my installation?

Thanks,
Piero