Hi julian, sorry for my delay response.
I changed the services as you suggested, essentially this was my first implementation.
I removed the file output.log, then I rebooted the system and started the services, when I went in the logs folder the file output.log was empty.
I used journalctl -fu nodebb.service to see the log of the service, and I discovered a nice thing:
UPDATE 2 [FIXED]
Okay, so after spending some times on this, I discovered what is the cause of the problem. Essentially, I have installed nodebb as root account (which is wrong, my bad), and the systemd service was configured to run nodebb as the user "sfarzoso", but that user was not the owner of nodebb folder, so when I started the service using systemd, a permission problem happen.
For fix this, I chowned the "opt" folder for "sfarzoso", which is the folder that I need to install nodebb, then I cloned the repo of nodebb and launched the setup again.
All works now.