It would perhaps be unreasonable to condemn so time-honoured a phrase. But it is generally better to say cannot or am not able than to use this faded metaphor. Perhaps it attracts because it seems to soften the blow of refusal; it is less abrupt than those little words. But against this must be set a certain foolishness that appears in the phrase when examined. "If you are not in a position to give me what I want, why not change your position?" is the retort that will come into your correspondent's mind.