macOS not allowing me to restore media files from one external drive to another I am trying to restore media from an external drive to another
  external drive now both the partitions have been formatted with the
  same partition map GUID partition map (I am also getting an APFS
  partition message on clicking the partition map option available under
  disk utility)). Now whenever I am trying to restore the partitions, I am coming across several errors.


This was the first attempt: I selected the main volume (as the source volume from which the media files are to be restored) which is at the top of the volume tree (Sandisk Ultramedia)

I am facing this error! (I have attached some screenshots to further explain my problem)

Error No.1




Error No.2

This time I selected the volume within the container APFS volume (as the source disk from where the media files are to be restored) which is the Sandisk USB External Drive/MacOS USB. But, again I got the same error.
This seems to be a bug with macOS Mojave. Even I tried to do the same. I came across the same errors you are facing. Luckily, I was able to clone the disks  which is same as restoring. 

There are several ways to clone a disk. You can use some utility tools available on and off the macOS App Store.

I tried this https://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html cloning utility to clone my main drive to an external SSD both having the same partition map - APFS. This should solve the problem.