Thank you for your support. Actually my config file didn't even have this configUSE_MUTEXES line. I added it but it's still returnig that error posted before.

Thank's again!!!

Which version of FreeRTOS.org are you using? Each source file has the version number at the very top. Versions prior to V4.5.0 did not include the mutex, but did include binary semaphores.

Regards.