As someone pointed out in another reply, "races" are buckets people are distributed into, frequently based on physical aspects. The point of saying "race is a social construct" is the nature of those buckets.

Northern Europeans, Southern Europeans, and Eastern Europeans (or rather people descending from those areas) differ in a number of physical aspects. Are they different races? For those who think race matters today, it seems not---they are all "white". Back in the heyday of scientific racism in the first half of the 20th century, they absolutely were---that is why the US had different immigration limits for different parts of Europe. Are native Australian people the same race as Africans?---there's no especially close genetic relationship as far as I know.

Physical differences exist. How you use those differences to divide people into groups and, more importantly, how you treat people of those groups is a social construct.