感谢您的邀请,但是由于时间安排,不能前往.请见谅 .
Thank you for your invitation,but I am not free then.I′m sorry for that.
希望对你有帮助O(∩_∩)O~
委婉一点的：
Thank you for your invitation,I′d like to come but the schedule doesn′t allow,I′m sorry for that.
商务的?
Thank you for your invitation,I′d like to come but the schedule has been full ,I′m sorry for that.
谢谢你的邀请,我很愿意去但是我的时间安排已经满了.