If you do not have access to a notebook, and you are comfortable typing on your phone, then you generally can journal on your phone, though, it is recommended to do it in an actual notebook.
One of the main reasons against journaling on phones is due to distractions. It is incredibly easy to get distracted with things related to our phones. Even if we put it in airplane mode and turn off all notifications, the fact that we are actively holding and using our phones for something is a doorway to distractions that can take you away from journaling.
Not to mention that typing for a while on a phone can get very frustrating for many people. Constantly having to erase what was written because you accidentally touched the wrong letter is also distracting, demotivating, and a cause of major frustration. It can just take you away from your train of thought and cause you to lose focus.
One of the greatest benefits of writing in an actual journal is the effect it has on the speed we think. Our minds are constantly racing from thought to thought, not really giving us enough time to actually focus on a single thought long enough to actually understand it well enough. Journaling, in the pen and paper form, causes us to slow our minds down in order to write down our thoughts, allowing us the time to process them, explore them, and understand them better.