A letter/email of apology to customer / contact who was seeking help for being unable to help because of out of office / conflicting schedule / time limitation.
Thank you for your message seeking assistance in relation to [event / matter].
[We are / I am] sorry that [I am / we are] unable to assist this time because [I / We] [will be travelling at that time] / [have an urgent matter to handle] / [have a prior engagement which we must attend].
accept our apology,
unable to help,
out of office,