You cannot access modern sites using SharePoint Designer. My question is so what's the solution for now until you come up with something? I needed to build this Workflow for now and SharePoint designer is the only tool we're allowed in our org, NO 3rd party tools or Flow/Powerapps, these products aren't yet allowed in most Gov sectors including mine. SO what now MSFT?I am just confused why they don't consider everyone and every business aspect?
Where did Microsoft said that? I think you mean that you should not try to open modern pages with SPD 2013, but for building classic workflows you need to use SPD 2013