Ok to say no

It is perfectly OK to say "no" as an answer.

Sometime, the best answer is "no".

"No" can be seen as rejection, negatif respond or decision. It does mean that way. 

However, "no" can sometime play as an important tool, path, way, to learn, to be creative (thinking other alternatives), to achieve a better result, to pivot and pass the previous target/goal.

No can be a gift.

Learn to use it. Please.