Make sure to read the shift description carefully before you apply. It includes what skills the client expects, as well as details like the preferred dress code. Skipping this step might mean you show up and can’t actually start working.
If you arrive and it turns out you don’t have the right skills (for example, if it’s a barista shift but you don’t know how to use a coffee machine) the client can send you home. They can also mark you as a no-show, which will affect your show-up percentage.
If you’ve already started working but the realize the tasks don’t really match your skills, talk to the person in charge right away. In that case you can decide together to finish the shift and you'll get paid for the exact hours you’ve worked so far.