What do I do about a compiler error " variable 'font' must be const"
The problem is you're using an old version of the Adafruit GFX Library, which was written for a previous version of the Arduino IDE. You just need to install the latest version of the library and your problem will be solved:


Download https://github.com/adafruit/Adafruit-GFX-Library/archive/master.zip
Unzip the downloaded file.
Rename the unzipped folder Adafruit_GFX
Replace /home/Harry/Arduino/libraries/Adafruit_GFX with the folder.


If you run into any other problems after doing that I recommend trying the latest version of the Arduino IDE. The version you're using is very outdated.