Syntax error: No operation specified when trying to resize Ubuntu VM

I'm trying to expand the hard drive of my Ubuntu VM in VirtualBox from my Macbook Air terminal. But when I type in

VBoxManage modifyhd <uuid>|~/VirtualBox VMs/Ubuntu/Ubuntu.vdi --resize 20000

I get this output:

Usage:

VBoxManage modifyhd         <uuid|filename>
                            [--type normal|writethrough|immutable|shareable|
                                    readonly|multiattach]
                            [--autoreset on|off]
                            [--property <name=[value]>]
                            [--compact]
                            [--resize <megabytes>|--resizebyte <bytes>]


Syntax error: No operation specified

But I specified the operation though, so I'm a bit confused.