If you see a message saying "Delta RPMs disabled because applydeltarpm not installed" when trying to update packages on a Red Hat or CentOS system, it means that the applydeltarpm package is not installed on your system.
Delta RPMs are a type of package that contain only the differences between two versions of a package, rather than the entire package. They are used to reduce the size of package updates and to improve the speed of package installations.
The applydeltarpm package contains the applydeltarpm utility, which is used to apply Delta RPMs to update packages on the system. Without the applydeltarpm package installed, the system will not be able to use Delta RPMs and will have to download and install the full packages instead.
To fix this issue, you can install the applydeltarpm package using the yum command. For example:
$ sudo yum install applydeltarpm
This will install the applydeltarpm package and you should no longer see the message about Delta RPMs being disabled.
Note that the applydeltarpm package is not installed by default on Red Hat or CentOS systems, but it is available in the default repositories. If you are unable to install the package, it may be because the repositories are not configured correctly or because there are connectivity issues. In this case, you will need to troubleshoot the issue and resolve any problems with the repositories or the network connection.