linux – Cannot install kernel core

I am trying to upgrade my CenOS from 7 to 8. Refer to www.tecmint.com/upgrade-centos-7-to-centos-8/ I find errors when doing step 5

Step 5: Install the New Kernel Core for CentOS 8 To install a new kernel for CentOS 8, run the command.

# dnf -y install kernel-core

Here is my situation.

[root@localhost ~]# dnf -y install kernel-core 
CentOS Linux 8 - BaseOS     0.0  B/s |   0  B     00:00 
CentOS Linux 8 - AppStream                      0.0  B/s |   0  B     00:00 
Failed to synchronize cache for repo 'baseos', ignoring this repo. 
Failed to synchronize cache for repo 'appstream', ignoring this repo. 
Last metadata expiration check: 1:22:03 ago on Wednesday, August 09, 2023 PM03:30:00 HKT. 
No match for argument: kernel-core Error: Unable to find a match: kernel-core

How to fix this? I cant find any solution about it. Thanks!