Create Proof Copy button missing

Based on the screen shots, it looks like GoldingD is on the right track.

The Create Proof Copy button is a shortcut for enabling the Settings > Proof command in the menu, and the Soft Proofing checkbox in the toolbar is also a shortcut for that menu command: All three are tied to the same setting, so they all change together. As already noted, the screen shot shows that what’s being edited is already a proof copy.

A little more background to get to the final explanation: All that a proof copy is, technically, is a virtual copy with soft-proofing enabled. When you click Create Proof Copy, all that does is make a virtual copy with the current soft-proofing settings enabled, so that is what you are looking at. Create Proof Copy is not available because you are already looking at a proof copy.

Back to your original question: If the Create Proof Copy button is not there because it’s already a proof copy, how do you make another one? The key is knowing that it’s actually a virtual copy. Duplicate that virtual copy by choosing the Photo > Virtual Copy command or pressing its keyboard shortcut. Now you have two proof copies (two virtual copies with soft-proofing settings enabled). If your goal was to save and compare proofs of different potential outputs, then you can now change the soft-proofing settings of the new virtual copy.

Doing that would leave you with the following three instances in your catalog:

• Original, with soft-proofing disabled to view its full tonal and color range. 
• Proof copy 1, targeting a specific output scenario. Your screen shot says the existing proof copy simulates sRGB color space with Perceptual rendering. 
• Proof copy 2, targeting another output scenario such as a specific printer, ink, and paper combination. 

Now, if all you really wanted to do is simulate a different output than “sRGB, perceptual” and you no longer need the current proof copy, then just change the Proof Settings of this virtual copy, so that you still have one original and one proof copy simulating the target output conditions.

To fill out the explanation, if you disable soft-proofing for a proof copy, then it is no longer a proof copy and falls back to just being a virtual copy. If you enable soft-proofing for the virtual copy, then that virtual copy becomes a proof copy again. So any virtual copy can be made into a proof copy by enabling soft-proofing for it.

You mean a shown in the following screenshot?

Do you mean the button shown by @Ian Lyons , or do you mean the checkbox to enable soft proofing at the bottom of the image? If you mean that the checkbox is gone, then you have hidden the toolbar. Press the 'T' key.

Please upload a screenshot showing the area of the UI where it should be. I've attached an example of what we need. You can use the 'Insert Photos' button to upload your screenshot.