Something like this?
df %>% group_by(N2000) %>% mutate(area = ifelse(N2000== "Yes",SurfaceN2000, Total_Surface )) %>% summarise(surface= sum(area, na.rm = T)) # A tibble: 2 × 2 N2000 surface <chr> <dbl> 1 No 16 2 Yes 5.5