What are you seeing in the CHIRP com port dialog box? It should be something like /dev/ttyUSB0 and you need to either be a member of the dialout group or change it to read/write with "sudo chmod 777 /dev/ttyUSB0."  The 777 is actually read/write/execute and is no doubt overkill but is guaranteed to work! Linux is the best OS for CHIRP,  it works even with counterfeit cables and no driver issues as in Windows and MacOS!