Thanks for your offer which, however, we are not in a position to accept because the date you have required doesn't match the availability of our service. Thanks very much again and looking forward to your next offer.
様々な言表現ができると思うのですが、ビジネスレター風に丁寧に書いてます。
依頼 an offer
~できない、立場にいない be not in a position to ~
希望する日 the date you have required
会わない don't match, doesn't match
Thanks for your offer which, however, we are not in a position to accept because the date you have required doesn't match the availability of our service.
Thanks very much again and looking forward to your next offer.