OK so there’s a slightly confusing step required to complete the process.
In Payroll Premier, after clicking Reports –> Single Touch Payroll –> Process STP Reports –> Financial Year 2020-2021, you will receive a JSON file looking a bit like STPEXPORT_20210706 1406 (EFY)
You upload this via the Reckon portal, using the second option “Upload a previously completed ATO Submission or Finalise Year”
Don’t let the “Previously Completed” line deter you – the grammar is a little unclear as it’s missing a comma.