Sometimes when opening mockups for the first time Photoshop needs to be reminded where the displacement map files are. Follow these 3 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: 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 3: 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 :).