This project requires a bunch of Arduino libraries. You will need to install all of these libraries to follow this guide.
Open up the Arduino library manager:
Search for the Adafruit GFX library and install it
If using an earlier version of the Arduino IDE (prior to 1.8.10), also locate and install Adafruit_BusIO (newer versions will install this dependency automatically).
Search for the Adafruit SSD1306 library and install it
Search for the DHT Sensor library and install it
Search for the Adafruit TSL2561 library and install it
Search for the Adafruit Sensor library and install it
Search for the Adafruit nRF51 library and install it
You'll need to install the Timer library manually. Download it from the following link and import it into the IDE with Sketch -> Include Library -> Add .ZIP Library
We also have a great tutorial on Arduino library installation at: