What am I missing? I cannot find a photomerge tool in Elements 2018?
That's already several PSE versions that Photomerge has been moved to the 'Guided edit' section of the editor instead of the 'Expert' mode. No change in the organizer where it is still logically in the menu Edit >> Photomerge (recommended).
I would like to have it on a disk and not the cloud.
Elements has never been "on the cloud". It's a 'perpetual' licence not a subscription one. You can buy a box or download the installer from your Adobe account. Be sure to have a look at the online help files, especially for the new features ("What's new in Elements 2018"). Ask here for more detailed support.