I have a project with the following file structure: led-test │ platformio.ini │ ├───lib │ │ readme.txt │ │ │ └───ht16k33 │ └───src │ ht16k33.cpp │ ht16k33.h │ └───src main.cpp And the following files: main.cpp #include <Arduino.h> #include <ht1…

Solution 1 Add #include <Wire.h> to one of the project file. Solution 2 See lib_deps . [env:teensy31] platform = teensy framework = arduino board = teensy31 lib_deps = Wire