Hi Melissa - Not sure why you are unable to access Reckon GovConnect? In order to do STP you need a GovConnect setup for this ABN - and this is when the Software ID is generated - that you accountant can add to the ATO?
If that ABN has been registered, its possibly been done by the accountant? so they would use the Software ID that is applicable to that registration? Or maybe you have registered it already on another Portal login?