The Arduino Due has an Atmel/Microchip Atmel AT91SAM3X8E microcontroller, which integrates an ARM Cortex-M3 processor with Ethernet, USB, CAN, USART, and other peripheral.  Since it's a Cortex-M3 processor, there is no MMU, memory management unit, to provide virtual memory capability.

Ubuntu is a Debian-based version of the Linux kernel bundled with Open Source software to provide a full-featured operating system with a graphical user interface (GUI).  The system hardware requirements for running such an OS include a MMU and a video interface, and the Arduino Due has neither.

However there is a version of the Linux kernel (derived from the uClinux project) that does not require a MMU.  There are also small-memory-footprint versions of the C library (uClibc) and user-space utilities (Busybox).  Combine these, and you can build/install a headless (no video/graphical interface, just a serial terminal) version of the Linux OS for a MMU-less SoC with limited memory.

See [Practical Advice on Running uClinux on Cortex-M3/M4]( https://www.electronicdesign.com/technologies/embedded-revolution/article/21795660/practical-advice-on-running-uclinux-on-cortexm3m4) for the limitations of such a system.