错在哪儿?

He has had a new bike for a week.
He bought a new bike a week ago.
buy买， 不是持续性动词， 所以不能用于完成时， 只能是过去式。
have则是持续性动词， 表示“拥有”， 可以用完成式

bought 不是延续性动词 。
改成he has bought a new bike a week ago .就对了。

he bought a new bike one week ago.

or

he just bought a bike.

买是个瞬间动作，所以不能接for a week.