Creating link between two tables

Hi

I have data model as below;

I have created a unique constraint as below;

CREATE UNIQUE NONCLUSTERED INDEX [UNQ_StaffIDEventID] ON [dbo].[tblStaffBookings]
(
	[StaffID] ASC,
	[EventID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON)

I now tried to create a link between two tables tblStaffBookings and tblEventStaffDisciplinary by dragging from tblEventStaffDisciplinary.StaffID to tblStaffBookings.StaffID and it gives me error as below;

What do I need to do to link tblEventStaffDisciplinary.StaffID to tblStaffBookings.StaffID?

Thanks

Regards