First its not ideal to use NTFS on linux as its Microsoft propietary and therefore not well optimized and with missing features if you compare it against a linux filesystem like ext4 or xfs. If you just want to use that USB HDD with PVE I would suggest formating it to something else like ext4 or xfs.
Second point is that you need to set the "is_mountpoint" option for a directory storage if it is pointing to a mountpoint. That can only be done using CLI with pvesm YourStorageName --is_mountpoint yes.