There are several possible culprits here:

1. In /etc/monetra/main.conf, your modem lines are pointing to the improper device. By default, Monetra points modem1= to /dev/ttyS0. If you do not have a modem, and are only using internet connection devices, you must comment out BOTH modem1= and modem1_Init=.

2. Monetra runs by default as the unprivileged user 'monetra'. You must give this user permission to access the modem. The easiest way to accomplish this is to set read and write permissions open for all users using:
   chmod a+rw /dev/ttyxx
   After doing that, attempt to start Monetra again, and you should be fine.

   For the security minded administrator, if Monetra is the only program using the modem, you may:
   chown monetra /dev/ttyxx
   Or if other processes also use the device, create a modem "group" and add the 'monetra' user as a member of that group, along with your other user accounts that may access the modem.