You can only submit a visa application after you have successfully completed your course and you have been officially notified by email through the University. If you make your visa application before successful completion of your course, it will be refused by UKVI.
You will need to make sure you meet the full visa eligiblity crtieria before you make an application.
Please note, you can only apply for the Graduate Route visa once.
See UKCISA for the full detailed advice which also contains a helpful webinar that you can view.