I have purchased the NFT of a piece of digital art. Am I able to commercialise the artwork or create derivative artworks?

As discussed here, one should not assume that ownership of the NFT is equivalent to owning the IP rights in the underlying digital asset. Whether you can commercialise the artwork or create derivative artworks will depend on the rights you have acquired as the NFT holder.

For example, the terms of sale may state that by purchasing the NFT, all IP rights in the underlying artwork will be assigned to you, or the terms of sale may only grant you a limited licence in relation to the artwork. In the latter case, you may only use the artwork within the scope of the licence granted. Therefore, if the licence is for non-commercial use, then commercial use will be in breach of the licence.