I could not upload a VM to Google Cloud so I contacted support. They examined the Cloud Endure log and answered the following:

"It looks like this kernel header package is missing on your source machine for a successful installation of the CloudEndure agent: kernel-devel-4.4.172.rpm.
Could you please install it and retry installing the agent?"

Where can I download kernel-devel? Can I install this package on my Slackware 14.2, with 4.4.172 kernel? I want to upgrade the kernel to 4.4.189, will I find the devel for both versions?

You have been poorly advised. There is no kernel-devel package in Slackware.
A quick web search found this link. It appears that the CloudEndure agent is installed using a Python script that builds a kernel module.
Slackware is not an sudo distro, so modify the suggested commands appropriately.

The closest Slackware equivalent is the kernel-source package.

But do listen to allend.

Could also need the kernel-headers package.

Customer service reading from a script. The last time I tried slackware it came with all the dev tools out of the box. The -dev or -headers for the kernel is to build the kernel (and kernel modules) from source. Basically /lib/modules/$(uname -r)/build needs to exist and point at something. And that something helps you build things against the current kernel without having to rebuild the entire kernel.

$ ls -l /lib/modules/$(uname -r)/build
lrwxrwxrwx 1 root root 37 Jun 19 17:16 /lib/modules/4.19.0-5-amd64/build -> /usr/src/linux-headers-4.19.0-5-amd64

Is what it points at on my debian buster install. OFC the last time I tried slackware was like version 14.2 and I had to use a debian kernel to get it to boot at that time. But it lacked the java openGL components so I couldn't game. And I gave up on it when I had to be actual root to build slackware packages. Which I don't recall being in RPM format, but it was a long time ago.

http://slackbuilds.org/

Might help you find what you're looking for, or make it. Mostly for not packaged in the distros repo options. Like cwm, a window manager that originates from the BSDs. While I dont' run slackware, I do use slackbuilds to help me remember odd URL names for git repos from the simpler name of the things I do remember.