Apps in dock cant be moved or changed in the dock.. why?

You are activating Force Touch on the Trackpad. Newer Trackpads are pressure sensitive and will perform another operation if pressed all the way down (Force Touch).

When you Force Touch on a Dock Icon, it opens the App in Mission Control, showing all open windows.

Either click softer when you wish to drag, or disable Force Touch in the Trackpad System Preferences.

Stuart Carter wrote:

When I click to move or remove an app from the dock using my TrackPad on the iMac the screen just changes with nothing on the screen and says "No Available Windows"

How do I change this or stop the trackpad from doing this?

I have included a video.https://youtu.be/Ox0BF3aJXd0

It is unclear if this is a universal issue here from your single example.

Is this a old problem, new problem, what changed ...?

Command Tab show all open apps HUD (Heads Up Display)

Keep in mind Command W closes an application Window, but does not necessarily Quit the app (Command Q)

The Dock has specific regions:

Control click on Dock icons opens a contextual menu with Options.

or

click and move in one motion, without hesitation...

If you click and hold down, you can immediately drag off the Application.

It is a preference setting, if you want to see "Recently" used apps or not

>System Preferences>Dock & Menu Bar

You can look at options in >System Preferences>Trackpad as well

The Dock on your Mac - Apple Support

Use an Apple wireless keyboard, mouse, or trackpad with Mac ...

Get to know the Finder on your Mac - Apple Support

Stuart Carter wrote:

When I click to move or remove an app from the dock using my TrackPad on the iMac the screen just changes with nothing on the screen and says "No Available Windows"

How do I change this or stop the trackpad from doing this?

I believe Barney-15E above has zeroed in on your issue:

• Dock: Force click an app in the Dock to access App Exposé. This shows you all open windows for that app.

If in expose' there are no open windows, then what you see in your screenshot is what you would expect.

Learn the difference between simply closing a window (red pill button) and quitting an app.

Not all apps work the same in this regard— some built apps will quit when a window is closed, but not all.

Command H will always hide a window/application to get it out of the way. The yellow pill button is awkward at best if not useless, this I would never recommend.

I have also found ForceTouch more of a nuisance than an asset— and disabled it immediately for more consistent results.