Noun which may take the genitive case particle 'no', Na-adjective (keiyodoshi), Noun
crucial; key; most important; main (point)
そう言えば、肝心要の部分を何も決めていなかった。Now that you mention it, I wasn't involved in the decision-making for the vital part.
Also written 肝腎要【かんじんかなめ】