Can a certification course or internship provide a valid letter of recommendation (LOR) for my application for a Master's?

Yes, a certification course or internship can provide a valid letter of recommendation (LOR) for your application, as long as the recommender is someone who can credibly speak about your skills, performance, and potential.

A mentor, instructor, or supervisor from your certification course or internship who has closely observed your work can write an effective LOR. This is especially valuable if it highlights skills and experiences relevant to your intended master's program.