Email shopify@directory.com.au and include your store's .myshopify.com address.
At https://your-store.com/apps/support. Add a link to it from your navigation or footer (Online Store > Navigation). It renders inside your theme, so it already matches your store.
Check Settings > Portal: if "guests can open tickets" is off, customers must sign in first. Also check the minimum message length; the form explains it, but very short messages are rejected.
Use Settings > Email > "Send a test email". Check the spam folder, and that the reply-to and notification addresses are right. You can also send from your own SMTP server so the emails come from your domain.
Order and customer context needs the read_orders and read_customers permissions. The rest of the desk works without them; customers can still pick their order on the portal because the storefront already knows it.
The store has reached its storage allowance (100 MB on Free). The Plan page shows what is used. Delete old tickets to free their attachments, or upgrade to Pro for 10 GB. Tickets and replies keep working either way.
Attachment files plus the ticket data itself (messages, notes, log). The Plan page shows the split.