The lead resident is asked to specify the share of rent for each resident before submitting the application. You should enter the individual amount for each resident, or click Split equally to share the rent equally, then click Save and continue to proceed.
All residents must have a rent share of at least £1. You can't assign any resident a rent share of 0 and if you do, you won't be able to proceed. If the resident won't be paying any rent, you should give them a rent share of £1 so that they can be submitted for referencing. This will be amended to £0 during referencing.
If the resident is under the age of 18, they should be archived as a resident from the application and should instead be added as a permitted occupier. Please read this article for details.