I have a classic software project, project EMU. I think EMU was created without a scrum board. Our company uses one master scrum board from which we prioritize tasks amongst several projects. I created a dummy project "Actall Master Project" to have a master company wide scrum board pulling in from 2-3 active projects at a time. I think update my filter for the board on that dummy project to pull in tasks from the currently active projects.
I could add the classic software project EMU to the filter for our master scrum board along side of the other active projects we groom tasks for out of our single backlog. But when I create a task and switch the project to project EMU, the sprint selection pull down box goes away. I can drag tasks from the shared backlog into our current company-wide sprint. But I can't select the sprint while creating a task for project EMU. I actually created a dummy sprint within the EMU project to see if that would enable the menu pick, but it did not. Screen shots for the Dummy project "Actall Master Project" which allows me to assign its tasks to sprints and the active EMU project which doesn't allow me to assign its tasks to sprints are below.

