There is a very common expression in Farsi that translates to something along the lines of "It's my bad luck that I wasn't there to enjoy your company" or "I will not have the good fortune to be there with you guys", and it's usually used as an act of being humble. I mean they are not "literally" unlucky not to be there, it's not like something very important is going to happen, they are just being polite. And of course, depending on the context, it can take any tense. Is there any expression in English that can be used in such situations? Do the ones I used seem natural to say?
To me (a 27-year-old American), that phrase sounds very, very formal. I could imagine a professor saying that to a student, or an employee of a financial institution saying that to an important client. But it sounds too formal for most business communication, and it's definitely way too formal to say to a friend.
For most business communication, I think I'd say: "Unfortunately, I won't be able to join you that day, but I do wish you all good luck!"
And to a friend, I'd say something like: "Sorry, I can't make it, but good luck!"