It is impossible. You must have at least two sides which are parallel to one another.
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trapezoids
All regular polygons with an even number of sides. Irregular polygons with an odd number of sides can have parallel sides. There are also non polygonal shapes that can have parallel sides.
You need at least one pair of parallel sides. Most trapezoids have that.
A shape with four or more sides can have two parallel sides.
There are many such shapes. Any polygon with an even number of sides can have opposite sides that meet the above requirements.