please help me to understand the meaning of this sentence.

退休养老金计划通常要求受益人购买一个固定金额的退休年金

退休养老金计划常常要求受益人购买一个固定金额的退休年金。

退休金发放政策往往是在受益人定期交纳了一定的养老费才可生效

Retirement pension schemes often require their beneficiaries to putchase a fixed dollar annuity on retirement. 退休金方案通常要求其受益人购买一个固定的美元退休年金。