On the student profile's Support Tab, each Primary SEN Need has to be unique for that particular SEN Provision.
For this student, there has been a change in the Primary and Secondary SEN Needs associated with the SEN Provision. Both former SEN Needs have end dates, but the new Primary SEN Need used to be the Secondary one and the system remembers the connection for this Provision.
To resolve this, it is best to place an end date to the SEN Provision of SEN Support then set up a new SEN Provision with a new Primary SEN Need.
This will then display as follows:
Recalculate the census and the Error will no longer appear.