Usando aspas você está retornando valores de texto - remova as aspas, ou seja,
=IF(AND(N5>=60,N5<=63.999),2,IF(AND(N5>=64,N5<=66.999),2.25,IF(AND(N5>=67,N5<=69.999),2.4,IF(AND(N5>=70,N5<=73.999),2.5,IF(AND(N5>=74,N5<=76.999),2.75,IF(AND(N5>=77,N5<=79.999),2.9,IF(AND(N5>=80,N5<=83.999),3,IF(AND(N5>=84,N5<=86.999),3.25,IF(AND(N5>=87,N5<=89.999),3.4,IF(AND(N5>=90,N5<=93.999),3.5,IF(AND(N5>=94,N5<=96.999),3.75,IF(AND(N5>=97,N5<=100),4,IF(AND(N5<=59.999),0)))))))))))))
.... embora você não precise de todos esses ANDs - isso deve funcionar para você
=IF(N5<60,0,IF(N5<64,2,IF(N5<67,2.25,IF(N5<70,2.4,IF(N5<74,2.5,IF(N5<77,2.75,IF(N5<80,2.9,IF(N5<84,3,IF(N5<87,3.25,IF(N5<90,3.4,IF(N5<94,3.5,IF(N5<97,3.75,IF(N5<=100,4)))))))))))))
ou com o LOOKUP
=LOOKUP(N5,{0,60,64,67,70,74,77,80,84,87,90,94,97;0,2,2.25,2.4,2.5,2.75,2.9,3,3.25,3.4,3.5,3.75,4})