If you're an Aston student, then almost definitely. You must have opted in to Aston Students' Union membership - although if you haven't, or are unsure, you can check on opt in on MAP. There is no charge to be a member of Aston Students' Union.
For the Full-Time, or Sabbatical roles, you must be prepared to undertake a year's full-time, paid work in the Students' Union. Students often do this as a Placement role, or in the year immediately after graduating, although theoretically any student can get involved.