A Tessellation is up down left and right shape with no GAPS or OVERLAPS or else that would be a Non Tessellation. A tessellation needs to have all gaps filled and there should be no overlaps if there is gaps and overlaps you would have gotten it wrong. A Non Tessellation is when you have gaps and overlaps when you are creating a shape.
What was most challenging was creating a shape putting it together and seeing if it will fit like with some of the pictures above. But the easiest way for me to make it was the online website making and making a real one out of the shapes and picturing in my head if it will fit.