There are various Online courses available.
however, the most popular is Digital and Social Media Marketing by Simplileaen.
you ca also go for Business Analysis course by University of Pennsylvania from Coursera as online mode.
A course in Machine Learning or Artificial intelligent is highly appreciated and most sought after by Marketing professional, you caa get such online courses in Edx and Udemy.
You can jojn for basic  Business Communication skills from Linkedin at very minimal cost.
Marketing has plethora of opportunities and above courses shall aid in Placements as well as college admissions.