The interviewer invited me for an interview this afternoon, but I was not available. What should I say?

I have carefully read your question, and my suggestions are as follows:

First of all, if the interview time agreed by the interviewer and you conflicts with your schedule, you should tell the interviewer truthfully and arrange another interview time with him.

The interviewer invites you to the company for an interview. The time and place of the interview must be agreed by both parties, so as to attend the interview on time.

If the interview time of the company is a hard requirement and cannot be accommodated or adjusted, then you can only choose by yourself.

I hope my answer can help you and make you satisfied.