I have a vbox extension virtual machine instead of vdi. How should I resize storage of vbox?

I’m trying to install Ubuntu on the virtual box using USB. My primary OS is also Ubuntu. When I’m booting, it’s giving me this problem: screenshot

So, I’m trying to find a way to resize the storage. I came across this command: VBoxManage modifyhd YOUR_HARD_DISK.vdi --resize SIZE_IN_MB , but this only works if the image extension is vdi.

In my system, under Virtual box image folder, it’s shows Ubuntu.vbox instead of Ubuntu.vdi.