Before being considered for Rent Supplement, you must have been renting privately and have been able to afford the rent for 6 of the previous 12 months, or have been assessed by the Local Authority as having a housing need.
You mentioned that you're on the housing list - has the local authority carried out a housing needs assessment on you yet? There can be a delay of several months, depending on where you live. Until that's done, and the local authority state that you are in need of accommodation, you won't qualify for Rent Supplement.
If your housing needs assessment has been completed and you are eligible, then re-apply for Rent Supplement, making sure to bring evidence from the local authority.