Go to School > Data > Export > Data Returns and check whether the ILR return is available there.
You may see the school census on this page, so check whether you do need to complete the ILR. Post-16-only schools complete the ILR, whereas secondaries or all-through schools with a post-16 complete the school census. It is not possible for both the school census and the ILR to be enabled on a single site.
If you are sure your school should complete the ILR and not the school census, we'll need to switch this on for you. Please contact the Arbor Support Team if supported by Arbor, or your Support Partner if not supported by Arbor.