Error messages sometimes #4

Arduino: 1.8.18 (Windows 10), Board: "DOIT ESP32 DEVKIT V1, 80MHz, 921600, None"

In file included from C:\Users\OSVALDO'S PC\Downloads\Embedded_UKF_Library-master\ukf_example2_imu\ukf_imu_engl\ukf.h:11:0,

             from C:\Users\OSVALDO'S PC\Downloads\Embedded_UKF_Library-master\ukf_example2_imu\ukf_imu_engl\ukf.cpp:93:

C:\Users\OSVALDO'S PC\Downloads\Embedded_UKF_Library-master\ukf_example2_imu\ukf_imu_engl\matrix.h: In member function 'void Matrix::vPrint()':

matrix.h:1291:13: error: 'Serial' was not declared in this scope

         Serial.print("[ ");

         ^

matrix.h:1298:9: error: 'Serial' was not declared in this scope

     Serial.println("");

     ^

C:\Users\OSVALDO'S PC\Downloads\Embedded_UKF_Library-master\ukf_example2_imu\ukf_imu_engl\matrix.h: In member function 'void Matrix::vPrintFull()':

matrix.h:1303:13: error: 'Serial' was not declared in this scope

         Serial.print("[ ");

         ^

matrix.h:1310:9: error: 'Serial' was not declared in this scope

     Serial.println("");

     ^

In file included from C:\Users\OSVALDO'S PC\Downloads\Embedded_UKF_Library-master\ukf_example2_imu\ukf_imu_engl\matrix.cpp:9:0:

C:\Users\OSVALDO'S PC\Downloads\Embedded_UKF_Library-master\ukf_example2_imu\ukf_imu_engl\matrix.h: In member function 'void Matrix::vPrint()':

matrix.h:1291:13: error: 'Serial' was not declared in this scope

         Serial.print("[ ");

         ^

matrix.h:1298:9: error: 'Serial' was not declared in this scope

     Serial.println("");

     ^

C:\Users\OSVALDO'S PC\Downloads\Embedded_UKF_Library-master\ukf_example2_imu\ukf_imu_engl\matrix.h: In member function 'void Matrix::vPrintFull()':

matrix.h:1303:13: error: 'Serial' was not declared in this scope

         Serial.print("[ ");

         ^

matrix.h:1310:9: error: 'Serial' was not declared in this scope

     Serial.println("");

     ^

exit status 1

'Serial' was not declared in this scope

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I keep having this error popping up. Not sure what to do.