What are some types of constraints for creating artwork?

Supplies, Creativity, and Ability/Training.

Explanation:

Constraints are things that stop you from doing something, they constrain your ability to work. If you don't have the supplies you need for a type of artwork you are constrained to what you do have. If you are struggling with creativity while you are trying to work, you will be constrained to what you can create. Finally, if you are not trained in a type of art or your ability is lacking, the piece will be constrained to your abilities.