Assigning cost resources to summary task is grayed out

Hi,

I am trying to assign a cost resources to the summery task which I created in resources sheet as "Cost" and gave a group name. I turn into gantt chart and try to assign the resources to the summary task. I noticed that the assign button is grayed out. rest of the resources are good to assign.

Please advice what is the fix for this issue.

I am using MSP 2016.

Thanks