Not only can you cancel but that can be considered backing out of a trade and a negative ref could be given.
Hiddenleafpokemon is a trusted trader however he has still not left me a ref. It has been about a month and 2 weeks. I have sent him multiple PM's for him to leave me a ref. I left him a positive refs. It has been about a month and 2 weeks. What should I do? And should I change the ref I gave to him?
Wait, we can inquire about someone not giving us a ref?
There is no rule that you have to leave a ref. And no you can not change the ref you gave him.
If he didn't then not everyone does do they? I know plenty do not leave refs. It is not required.