Android permissions error can happen if the USB device doesn't exist (no driver, no usb cable attached, etc.) or, there is actually a permissions error in the /dev/ folder.

Do you see the /dev/ttyUSB0 file?

Use the following commands to check for the serial port:

$ adb shell 

$ ls -l /dev/tty*

You should see /dev/ttyUSB0 or similar. If not, the usbserial driver may not be loaded properly for this device.

Show me the drivers!

$ lsmod

You want to see usbserial.

Even if you see usbserial, it's possible usbserial is loaded up for the cell modem, instead of FTDI generic usbserial devices. Try refreshing the driver. 

Refresh the usbserial driver

$ rmmod usbserial

$ insmod /etc/firmware/usbserial.ko