I only want one "Display" screen

I'm not sure, but it may be Stage Manager, or the multiple Desktop feature...I guess this latter one is called Spaces. I don't use either one.

Organize your Mac desktop with Stage Manager - Apple Support

I guess multiple desktops is called Spaces as I found referenced in this thread:

How can I disable multiple desktops in Si… - Apple Community

Within that thread, I did find the following Apple article regarding Spaces (aka multiple desktops) since I could not find the reference directly:

Work in multiple spaces on Mac - Apple Support

There are some settings in System Settings > Desktop & Dock that you may want to check, and possibly turn OFF.

Including "Drag windows to screen edges to tile" and "Drag windows to menu bar to fill screen".

The other replies seem to have overlooked the fact that you are on High Sierra. You can disregard the mentions to "Stage Manager", which only exists in very recent versions.

What you have now are two "Spaces" (also sometimes called "Desktops").

You can delete the extra Space using Mission Control.

There are usually keyboard or mouse shortcuts to enter Mission Control, but they vary from mac to mac.

You can however do this: press Command-space and starting typing Mission Control until it gets recognized, then press enter to start it.

A screen showing all your windows and having at the top the existing Desktops will show.

Hover the mouse near Desktop 2 and click the x that appears to delete it.

Then press Escape to get out of Mission Control.

Luis Sequeira1 wrote:

The other replies seem to have overlooked the fact that you are on High Sierra. You can disregard the mentions to "Stage Manager", which only exists in very recent versions.

What you have now are two "Spaces" (also sometimes called "Desktops").

Thanks for noticing and correcting that oversight.