お力になれなくて残念です。
Ochikara ni narenakute zannen desu.
I regret that I can’t be of any help.

When you are presented with a request from someone to whom you’re not particularly close, you can use this expression to politely decline, if you wish. A similar alternative expression is:

お役に立てなくて残念です。
Oyaku ni tatenakute zannen desu.
I’m afraid that I won’t be able to be of any assistance.

If someone asks you to take charge of a neighborhood project or to accept some other position of responsibility, you may well feel you’re not qualified for the job. In such cases as this, a popular strategy is to plead a lack of ability:

私には荷が重すぎて……。
Watashi ni wa ni ga omosugite …
I don’t have what it takes.

力不足で、とてもご要望にお応えできそうにもありません。
Chikara-busoku de, totemo goyoubou ni okotae dekisou ni mo arimasen.
I just don’t have the ability—I couldn’t do a satisfactory job of it.

A similar gambit is to point out the trouble that would likely result from rashly accepting responsibilities that one is ill-equipped to perform:

せっかくお声をかけていただきましたが、なにぶん仕事が忙しく、お引き受けしてもかえって皆さんにご迷惑をおかけするだけですので……
Sekkaku okoe o kakete itadakimashita ga, nanibun shigoto ga iso-gashiku, ohikiuke shite mo kaette mina-san ni gomeiwaku o okake suru dake desu no de …
I appreciate your asking me, but I’m already so busy with work that if I were to accept I’m afraid I’d only end up causing everyone a lot of trouble.

　Some situations call for more a forceful way to turn down a request. Let’s say that you have shown an interest in or even tentatively agreed to a request, only to find that the favor asked has suddenly grown bigger or more complicated than anything that was originally discussed. Your response is to emphatically opt out, but rather than imply that anyone was trying to take advantage of you, you might simply say:

この話はなかったことにしてください。
Kono hanashi wa nakatta koto ni shite kudasai.
Let’s just forget that this subject ever came up.

This expression conveys a strong and unmistakable rejection of the proposal. Since the situation is not what you were led to believe and no further discussion will make it so, you can use these words to put an end to the subject. A sightly different but related expression is:

おろしてもらいます。
Oroshite moraimasu.
Deal me out.

When you have previously agreed to a request but later wish to withdraw your consent, you can use this expression, though it won’t in any way shield you from hard feelings on the other side.

– Source: A handbook of common Japanese phrases –