Convert 3d object to 2d

Hello, I'm fairly new to photoshop and have a very basic understanding of the program. Recently I was doing mockups for school that I downloaded from this website called freepik.com. On that website they have tons of mockups to download and all you have to do is select the layer and replace the photo with your own photo of the same size. However, a problem I'm having is that I downloaded a mockup not knowing that it contained a 3D object. So, I cant just change the photo because there is a 3d cube icon instead of the picture icon on that layer. I'm wondering if it is possible to somehow convert that 3D object to a 2D object so that I can place my photo in this mockup? It's also giving me all sorts of problems, telling me my scratch disks are full, which I don't know what that means and I can't move any of the layers. Also, it gave me a notification stating that photoshop would be getting rid of the 3d feature, so I'm wondering if it's possible to get rid of the 3d element all together. If not, are there other ways that I can create a mockup myself? Thank you so much!