Problem: This text seems to indicate that human longevity after the flood will not exceed “one hundred and twenty years.” Yet in Psalm 90 Moses took it to be as 70 or 80 years at best (v. 10).
Solution: First of all, it is not certain that Genesis 6:3 is referring to human longevity. It may be speaking about how many years remained before the flood would come.
Second, even if it does envision how long humans would live, it does not contradict the later reference to 70 or 80 years for two reasons: for one, it refers to an earlier period when people still lived longer (Moses himself lived to 120, Deut. 34:7); further, the 70 or 80 was probably not intended as an absolute upper limit, but merely as an average for people who died of old age.