You now probably on you first page, gave the microflow a parameter of type object and therefor the selectionofObjects is not a viable input for that microflow.
I think you did that after you ran into the problem that you cannot pass a listofobjects to a page, but only an object.
Solution is to create an helperobject and associations from that object to the selected Objects, store that helperobject in the database and pass the helperobject to the page. On that page you should have a dataview with datasource ‘content’ an in it a datagrid with the selected Object over association to the helperObject.
and the microflow:
For a working example: mydemoversion8-sandbox.mxapps.io in block Entities go to Employee and button “Action” behaves conform your requirement.
I do hope that there is a better way though….