请问，按照视频改写的 pom.xml

提示错误信息是：

Project build error: Non-resolvable import POM: Failure to find org.springframework:spring-framework-bom:pom:$(spring.version) in http://maven.aliyun.com/nexus/

 content/repositories/central was cached in the local repository, resolution will not be reattempted until the update interval of alimaven has elapsed or updates are forced

注释掉 <type>pom</type> 则不报错，请问原因。

谢谢！