When including Timer.h from /usr/local/frc/include/frc, Timer.h attempts to #include frc2/Timer.h, but /usr/local/frc/include/frc2 is missing.
If frc2 is copied into place from /usr/src/allwpilib/wpilibc/src/main/native/include/frc2, then it includes units/time.h which is missing as well.
When including Timer.h from /usr/local/frc/include/frc, Timer.h attempts to #include frc2/Timer.h, but /usr/local/frc/include/frc2 is missing. If frc2 is copied into place from /usr/src/allwpilib/wpilibc/src/main/native/include/frc2, then it includes units/time.h which is missing as well.