We ask that your references be professional. This can be a supervisor, volunteer coordinator, professor, or anyone else who has supervised you in any capacity.
Professional references are strongly preferred because they shed light on important areas like your willingness to take feedback, your ability to learn quickly, and the way you work in a team.
If none are available (maybe it's been a while since you were in a workplace - that's totally fine!), feel free to use personal references who can speak to your character and empathy.