Cut Trace on Pin #18
In order to properly use Pin #18 (left button) we'll need to cut the trace on the back of the PiTFT. Using a sharp hobby knife, you can score in between the pads to cut the trace.
I recommend testing the connection with a multimeter using the continuity mode. Press the probes onto the pads. If the multimeter beeps, then the trace isn't completely cut. It's best to ensure the trace is cut now than later in the build.