Well "No." might be a complete sentence and enough to express unwillingness to proceed with the demand. But to my mind, as others already noticed here, it is not the most productive way to deal with refusal, especially if quality of relationship is taken in consideration.
There are already plenty of great resources to answer negatively to the demand without damaging the relationship or the image that will be forged out of the way you behave, so just go and search for "how to say no"[1]
[1] https://duckduckgo.com/?t=ffab&q=how+to+say+no&ia=web for the lazy, and here are the first result as I searched it:
- https://www.indeed.com/career-advice/career-development/how-...
- https://www.wikihow.com/Say-No
- https://www.scienceofpeople.com/how-to-say-no/
- https://www.grammarly.com/blog/saying-no/
From my readings on this topic, the main take away things I liked were formulations that
- use positive assertive sentences: focus on what this refusal allow you to do, propose alternatives
- make it clear why you will focus on something else: it can also gives clues on whether it’s likely a permanent case or a local agenda/load conflict
- use the right level of kind firmness
Here is an example to illustrate what I mean:
"Thanks for your invitation at this restaurant, saying 'yes' would have delighted me, unfortunately I already have an other dinner planned at this date. Also I adopted a vegan regime, so what about going to Vegood the next Friday?"
First, be kind, the person asking you is making some kind of recognition of your existence and possibly bet on your skill set. Secondly, making a word like "yes" appears in the sentence make the refusal psychologically easier to perceive as some positive response. Then there is first an exposition of a some incidental impossibility: a typical conflict of agenda. But there is also this more perennial constraints about what you can it (whether you chose the regime for ethical reasons or health imperative necessities). Finally you propose an alternative.
Of course the alternative could also be some delegation on someone else than your future self: "My skill level on this technology would prevent me from achieving the task in due time. I think Ada would be way more qualified than me for this mission, or if she’s too busy, maybe ask Zeno: if he can’t help you directly, he is well connected with all experts in this domain."