Your device may have a memory problem. If you have managed to generate the right foot, re-open the application and then choose the option to continue with the process. You should be able to finish capturing the left foot without problems.
If the application closes before you have obtained the 3D images of the right foot or you are not able to complete the process, try closing all the applications you have open and try again.
If the problem persists, you should update your operating system to the latest version.