Sometimes when opening mockups for the first time Photoshop needs to be reminded where the displacement map files are. Follow these steps to update the location. Be sure to save your work :).
Step 1: Select the Displacement Effect
Double click the “Displace” effect. It's located inside Effects > Design > Displace.
Step 2: Confirm Settings
Click OK on the prompt. The default displacement settings will work well. If you want to customize the appearance you can try setting the Horizontal and Vertical scale between 5 and 15. Make sure both settings are the same. For example 5 and 5, or 8 and 8.
Step 3: Locate the displacement map
Find the displacement maps inside the folder you downloaded. Select the one that corresponds to the file you're editing.
Step 4: Save your changes
This one is important. Save your updated file so Photoshop remembers where to look the next time it's opened.
That's it! Have fun :).