I convert my 2 internal partitions in APFS in recovery mode (to convert the boot partition that was not allow by the Beta installer due to HDD drive). Then in normal mode, I try to use Disk Utility to convert my external USB drive (normal HDD drive, not SSD), and it failed (com.apple.StorageKit erreur 118).
Find and click the Convert to APFS option from the drop-down menu. Click the Convert button from the message prompt to confirm the process. Wait for the conversion process and click on the Done option after the process. You can also opt to select the current startup disk and click on the Erase option at the top portion of the screen.