Why is my sugar paste becoming a brick? I know this question may sound to be not inherent with chemistry, but since making the sugar wax paste involves acids and heating, I thought this would be the right place.

Preparing the paste is quite simple:
1 cup of sugar, 2 tbs of lemon juice and 2 tbs of water. Then the process expects to heat the compound until it becomes abra colored. The the mixture is moved away to cool down.The result would be a malleable and sticky paste at ambient temperature. My problem is that my paste becomes really hard and impossible to shape.

What could I do to avoid the compound to become a brick?
You don't have enough liquid in the recipe. Double the lemon and water to 4 tablespoons each, and you should have better luck. Adding more liquid will bring the sugar/liquid ratio more in line with what is needed to make the paste pliable. Add more liquid if you still have issues.