Welcome to the site. Before I get to the answer, I do have a question. Whereabouts in Australia? I travel there frequently (well, until the borders were shut down) for work. I have employees there, specifically in the Brisbane area. I love that city.
Asto the answer. There needs to be a clarification as you are using two conflicting terms. A subset is part of the larger main checklist usually focusing on a specified topic. An insert set, which this sounds like, are a distinct set separate from the main checklist. As this sounds like an insert set it the process would be easier. Someone will need to create the set and then a checklist will need to be uploaded. Seeing that you probably don't have the permissions yet you can enter an Inaccuracy Report. On the set's page, on the left panel you'll see this option. Frmo there you'll see n option to enter a missing set.
Before doing so you'll need a COMPLETE checklist. Specifically the insert set's name, correct card numbers, full name of the player as seen on the card and the team name for that player. This information will help someone enter the checklist for you.
My sins have come to face me, I can feel it
That I have lived my life in vain
And now I know I'll reap the seeds I've sown