You could try using --skip-broken to work around the problem Found 6 pre-existing rpmdb problem(s)

已加载插件：fastestmirror, product-id, subscription-manager
This system is not registered to Red Hat Subscription Management. You can use subscription-manager to register.
设置安装进程
Loading mirror speeds from cached hostfile
http://nginx.org/packages/rhel/4/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - “The requested URL returned error: 404 Not Found”
尝试其他镜像。
To address this issue please refer to the below knowledge base article

https://access.redhat.com/articles/1320623

If above article doesn’t help to resolve this issue please open a ticket with Red Hat Support.

解决依赖关系
–> 执行事务检查
—> Package nginx.x86_64 1:1.14.2-1.el7_4.ngx will be 安装
–> 处理依赖关系 openssl >= 1.0.2，它被软件包 1:nginx-1.14.2-1.el7_4.ngx.x86_64 需要
–> 处理依赖关系 systemd，它被软件包 1:nginx-1.14.2-1.el7_4.ngx.x86_64 需要
–> 处理依赖关系 libcrypto.so.10(OPENSSL_1.0.2)(64bit)，它被软件包 1:nginx-1.14.2-1.el7_4.ngx.x86_64 需要
–> 处理依赖关系 libc.so.6(GLIBC_2.17)(64bit)，它被软件包 1:nginx-1.14.2-1.el7_4.ngx.x86_64 需要
–> 处理依赖关系 libpcre.so.1()(64bit)，它被软件包 1:nginx-1.14.2-1.el7_4.ngx.x86_64 需要
–> 完成依赖关系计算
错误：Package: 1:nginx-1.14.2-1.el7_4.ngx.x86_64 (nginx)
Requires: libcrypto.so.10(OPENSSL_1.0.2)(64bit)
错误：Package: 1:nginx-1.14.2-1.el7_4.ngx.x86_64 (nginx)
Requires: libpcre.so.1()(64bit)
错误：Package: 1:nginx-1.14.2-1.el7_4.ngx.x86_64 (nginx)
Requires: libc.so.6(GLIBC_2.17)(64bit)
错误：Package: 1:nginx-1.14.2-1.el7_4.ngx.x86_64 (nginx)
Requires: openssl >= 1.0.2
已安装: openssl-1.0.1e-57.el6.x86_64 (@base)
openssl = 1.0.1e-57.el6
错误：Package: 1:nginx-1.14.2-1.el7_4.ngx.x86_64 (nginx)
Requires: systemd
You could try using --skip-broken to work around the problem
** Found 6 pre-existing rpmdb problem(s), ‘yum check’ output follows:
PackageKit-0.5.8-21.el6.x86_64 has missing requires of PackageKit-yum = (‘0’, ‘0.5.8’, ‘21.el6’)
PackageKit-0.5.8-21.el6.x86_64 has missing requires of PackageKit-yum-plugin = (‘0’, ‘0.5.8’, ‘21.el6’)
anaconda-13.21.215-1.el6.x86_64 has missing requires of anaconda-yum-plugins
anaconda-13.21.215-1.el6.x86_64 has missing requires of yum-utils >= (‘0’, ‘1.1.11’, ‘3’)
python-urlgrabber-3.9.1-11.el6.noarch is a duplicate with python-urlgrabber-3.9.1-9.el6.noarch
rhn-check-1.0.0.1-16.el6.noarch has missing requires of yum-rhn-plugin >= (‘0’, ‘0.9.1’, ‘35’)
解决方法：
yum clean all
rpm --rebuilddb
yum update
执行上述三条命令即可