OpenCL will not work if your user is not part of the group Video.
Open a terminal window by pressing Ctrl + Alt + T keys on the keyboard, then add your user to group Video:
Restart your PC:
Open a terminal window by pressing Ctrl + Alt + T keys on the keyboard, then execute the following commands:
This will print all available information about all OpenCL platforms on the system and the devices they expose.
In case the number of platforms is equal to 0 you need to install the OpenCL driver.