How to fix the “Smart Object is not directly editable” error: Option 1 is to right-click the Smart Object layer and click “Rasterize Layer”. This converts it into a layer of pixels that can now be edited. This technically solves the problem, but it may create a new problem for you — because this removes the Smart Object functionality from that layer, meaning you can no longer replace its contents to create mockups.
If your ultimate goal here is to change the appearance of the mockup scene while still being able to use the Smart Object to make mockups, one solution is to add a layer on top of the Smart Object, and apply your desired changes to that new layer. Also note that you can apply Effects to Smart Object layers without rasterizing them, and you can also create Adjustment Layers whose changes will only apply to the Smart Object layer if you link them via a Clipping Mask. If you’re trying to modify the item inside of the Smart Object, you can double-click the Smart Object layer to open it in a new tab. Make your changes, then save this tab, to update the main document.
Once everything looks good, if you’re using this document to create a large number of mockup images, consider automating that process using the Batch-Replace Smart Objects plugin. This Photoshop plugin will save you tons of time by generating all your mockups in bulk.
