Both master and detail controls must be bound to groups that are part of the same repeating group in the data source. Alternatively, ...

Both master and detail controls must be bound to groups that are part of the same repeating group in the data source. Alternatively, neither control can be bound to a group that is part of a repeating group. You can do one of the following:

To change the binding for either control, right-click the control, and then select Change Binding.

In the Fields task pane, move the groups to which the master and details controls are bound into the same repeating group. Alternatively, move both groups out of a repeating group.