You know you cannot attend but delay answering because declining feels awkward. The organiser keeps a place for you and sends another message.
Thank them for the invitation and say that you will not be able to join. Give a brief reason if you want to, without inventing an elaborate explanation. If a date or payment depends on your response, answer within the requested period.
Offer only what you mean
Suggest another meeting or form of participation when you actually want and can arrange it. A vague promise to catch up soon can leave another unfinished expectation.
If you are uncertain, say when you will know and ask whether the organiser can wait. They may need a definite answer sooner. Once you decline, avoid reopening the decision repeatedly unless circumstances genuinely change. If you later become available, ask whether there is still room rather than assuming the original place remains yours. A clear response gives everyone a usable plan while leaving the relationship room for another occasion.
