How to install the Gimp normal maps add on for Ubuntu 18.04?

The page has a Download gimp-normalmap section. Since you are likely running a 64-bit Linux you click on the "amd64"(*) link in the "architecture" column.

That page in turn tells you how to install an additional repository (recommended option). If you do so, sudo apt update (to refresh repository info) followed by sudo apt install gimp-normalmap will install the plugin (with the right architecture for your installed system).

If you want to be more "manual", click any of the mirrors to download the .deb file from it. Then right-clicking the .deb in any GUI file manager will likely have an option to install it.

Side note: Gimp 2.10 is a major overhaul (IMHO it should have been named 3.0). On the whole, it is much better/powerful than 2.8 but still has plenty of little quirks, that will eventually be ironed out. Compatibility of some plugins isn't guaranteed. 2.9 was a development version, and as such has more debug features, isn't performance enhanced, still misses some functionality, and will complain about the use of any deprecated stuff. No point using 2.9 if you are on 18.04.

(*) "amd64" is just the name of the architecture for 64-bit Intel/AMD processors because AMD invented it first. It runs on processor from both manufacturers.