If you aren't able to seat students following these instructions, there are two things to check.
Step 1 - Check you have entered the students correctly
In Arbor for linear qualifications, you need to enter students into both the Unit and the Award if they are taking the whole Award. If you don't enter students into the Units they are taking, you won't be able to seat them for the specific Exam or Assessable.
Follow these instructions to enter them into the Unit: Entering candidates in Arbor
The information will be sent to the Awarding Organisations correctly when you make your submissions.
Step 2 - Check you have allocated students to the exam room
You won't be able to select a seat for a student if you haven't yet assigned them to a room. Follow these instructions to assign them to a room: Allocate candidates to an exam room