Do I have to have a wall at both sides to form a wardrobe

No, it is possible to create a fitted wardrobe with something called an end panel.  This sits at the end of the wardrobe where there isn’t a third wall to build against.  This means we can add fitted wardrobes to almost any space.

Example 1:Wall to wall.

Example 2:Wall to end panel.

Example 3:End panel to end panel.