Then you need to either download the ISO again, copy it to installation media again, or both. Keep doing it until the check succeeds.
In Linux, though you have a partition/device connected to the system, the file system is not accessible until you attach it to the root file tree. To access the file system, you mount it, or at least that’s what we call it because the command to do so is mount . You can use the “KDE Partition Manager” in “System Tools” to unmount your partitions and then everything should work correctly.