What you need to know
1) The startup drives of Macs are automatically converted from Mac OS Extended (HFS Plus) to Apple File System (APFS) when upgrading to macOS 10.13 High Sierra (When internal startup disk is a SSD / Flash Disk.) & macOS 10.14 Mojave.
2) DiskWarrior 5.2 is NOT currently able to rebuild startup disks using the new APFS.
3) This only applies to Macs running macOS 10.13 High Sierra or later.
4) Diskwarrior 5.2 can rebuild an external disk (including Time Machine volumes) using Mac OS Extended (HFS Plus) even if your startup disk is using APFS.
Please follow these steps if you already own DiskWarrior 5.2. Skip to the next section if you have not yet purchased.
1) Launch the DiskWarrior app.
2) Select your startup disk from the list of disks. The startup disk will usually be named "Macintosh HD."
3) Look for the text circled below. If the "file system is a newer version" and is labeled as "Apple File System" then your startup disk is using APFS and is not currently rebuildable.
Please follow these steps if you have not yet purchased DiskWarrior 5.2.
1) Click the Finder icon in the application dock. This will make the Finder the frontmost app.
2) Choose Computer from the Go menu (Go->Computer).
3) A Finder window will open with the disks connected to your Mac. The startup disk will usually be named "Macintosh HD." Select the startup disk and then click and hold the gear button in the window's toolbar. Choose "Get Info" from the menu that appears.
4) Look for the text circled below. If the format is "APFS" then your startup disk is using Apple File System and is not currently rebuildable.