ZFS is a file system, similar to EXT4, so technically, you can use it. However, for ProxmoxVE, I believe you might not find this option in the GUI. You might have to configure it manually. If it's your main root partition, I suggest installing the Debian OS first with EXT4, then ProxmoxVE on it. Afterward, create the ZFS pool manually and use it later in ProxmoxVE storage.