First step is to make sure the gatttools command is working…
Just open a shell on the pi and enter the command directly, ie.
gatttool -b MAC-Lightpack --char-write-req --handle=0x001c --value=31
If that doesn’t work you have an issue with your BLE connection. Either move the pi and lightpack closer together or use another pi.
If you’re not sure if BLE is working on your pi, check with a hcitool lescan:
sudo hcitool lescan
Check that it finds your Lightpack and the MAC is correct.
If it’s not a BLE problem need to check the Exec command is working. Do you have the exec binding installed? If you do and it’s not working, check through this thread.
Let me know how you go 
PS- is there anything in your logs? (openhab.log)