That is because it is improperly configured. Hit ESC at boot and find out what boot number your USB synoboot.img is. In my case it is number 7.
edit your {VM_ID}.conf using vi or nano /etc/pve/qemu-server/{VM_ID}.conf
should look something like this when complete:
args: -device 'piix3-usb-uhci,addr=0x18' -drive id=synoboot,file=/var/lib/vz/images/100/synoboot.img,if=none,format=raw -device usb-storage,id=synoboot,drive=synoboot
boot: 7
cores: 2
ide2: none,media=cdrom
memory: 4096
name: NAS-X2
net0: virtio=EA:5C:48:D3:D9:D5,bridge=vmbr1,tag=1010
net1: virtio=6A:53:32:11:A8:9B,bridge=vmbr2,tag=1020
net2: virtio=46:02:AF:FA:F6:F0,bridge=vmbr2,tag=1060
numa: 0
onboot: 1
ostype: l26
sata0: /dev/disk/by-id/ata-SAMSUNG_HD203WI_S285J1RZ601531,backup=0,cache=writeback,size=1953514584K,serial=S285J1RZ601531
sata1: /dev/disk/by-id/ata-SAMSUNG_HD203WI_S285J1RZ601548,backup=0,cache=writeback,size=1953514584K,serial=S285J1RZ601548
sata2: /dev/disk/by-id/ata-SAMSUNG_HD203WI_S285J1RZ601803,backup=0,cache=writeback,size=1953514584K,serial=S285J1RZ601803
sata3: /dev/disk/by-id/ata-INTEL_SSDSC2BA400G3_BTTV321000E5400HGN,backup=0,cache=writeback,size=390711384K,serial=BTTV321000E5400HGN
sata4: /dev/disk/by-id/ata-MKNSSDCR60GB_MK140429AS1354202,backup=0,cache=writeback,size=58615704K,serial=MK140429AS1354202
sata5: /dev/disk/by-id/ata-MKNSSDCR60GB_MKN1502R000080784,backup=0,cache=writeback,size=58615704K,serial=MKN1502R000080784
scsihw: megasas
serial0: socket
smbios1: uuid=7372bb90-1200-4c79-87cb-5b50379cacb5
sockets: 1
Note: The serial0: socket and sockets: 1 allows you to connect to the vty from proxmox server qm terminal {VM_ID}