First you have to install python. You have to install pywhatkit module to run this file.
For Windows In Command prompt type as "pip install pywhatkit" and hit enter.
For Linux also Type as "pip install pywhatkit" in Terminal.
Then edit the mobile number, message and time as you want. After that run that .py file. It will open the web.whatsapp.com browser tab and send the message as you mentioned in that file.