Replace “is” with “are”. Plural verb should be used here …that ‘are’ essential to achieve this objective. Also do not get confused with “not only” case, there is no error in this. Case of ‘not only’ ‘but also’- When but is included you can either add ‘also’ (or its alternatives) or not; both forms are common and standard. Hence the parentheses in but (also), which could also be written as (but) also, since but sometimes doesn’t appear either.