help me about this i am trying hard to solve this but unfortunately not kindly anyone help me
Have you deleted any cell (or code) or changed any prewritten code?
Hey @Muhammad_Hamza_Tufai,
Welcome, and we are glad that you could become a part of our community 
Perhaps, you might have missed out on running the initial cell in the notebook. Please note that every time you open the notebook or restart the notebook, you have run the cells starting from the first cell.
Cheers,
Elemento
thanks for all who guide me. i have done my course