Are you using the Modbus capabilities built into Classicladder or are you using some other Modbus interface in Linuxcnc, such as Mb2hal?
If you are using the Modbus facilities built into Classicladder, on the Modbus Configuration screen, In the far right column you assign the variable number used for your write coils assignment. It must be an integer number between 0 and what ever you set as the maximum number of bits (default is 10 or 20). (I'm not sure if this number is just bits or if it is hal output bits.)
So if you configured it for 8, for example when you use coil %Q8, Classicladder will write the value for that to the modbus coil.