Learning to be able to hear "no" or "not now" or even "later" is a very essential skill for living! We can't always get what we want, and when we want it, so being able to wait for things we want, and hearing the word 'no' is something we focus on ensuring our learners in our Great Start Programs, are able to easily do.
It can a very difficult concept for some people to understand - young children, people with autism, and people with intellectual disabilities. Often if you say "no, we can't have that now" they may not understand 'no' doesn't mean forever, it just means right now.
It is best to teach this skill early, and frequently! Find ways throughout the day where you can say "no we can't have that now" but you can offer something that is similar, and very highly preferred, so in this instance, hearing the word 'no' and not getting the thing you want right now, is second to getting something you are really quite happy to get! And if the learner is able to accept this new item/object/activity, without any objections, we would want to give them LOTS of it (or lots of time doing it), so they realise that "hey, hearing 'no' isn't that bad!"
It can also be useful to teach someone to 'choose something else' - again linking it to highly preferred things that they would generally be happy to have. Not only does this help them accept hearing 'no', but it teaches a strategy they can use in the future as well - choosing other options.
As always - start small, and set your learner up for success! Accepting hearing 'no' can be something very difficult to learn, but we all are told 'no' at some stage in our lives. So look for everyday opportunities to find ways for your learner to practice accepting hearing 'no'.