Why could Boaz not redeem Ruth right away?

Answer: A closer relative had first right

Boaz said, "There is a redeemer nearer than I," and promised to redeem her himself if that man would not.

“Yes, it is true that I am a kinsman-redeemer, but there is a redeemer nearer than I. Stay here tonight, and in the morning, if he wants to redeem you, good. Let him redeem you. But if he does not want to redeem you, as surely as the LORD lives, I will. Now lie here until morning.””— Ruth 3:12-13 (BSB)