boolean is a type defined within the first few lines of
typedef bool boolean;
bool is a primitive C++ type. If your IDE can’t resolve that type (highlight the type, right click, “Go To Definition (F12)”), make sure you have
#include <Arduino.h> as the first line of your file. You might need to re-build your project index (by executing the build target
PLATFORMIO_REBUILD_PROJECT_INDEX) or rebuild your IDE’s C/C++ index by closing and re-opening the project. If none of that works you need us to show the code in which the Intellisense fails to resolve that type.