Answering questions about a lack of experience in a particular area can be a bit tricky, and it's wise to consider your answer ahead of time, since this question will very likely arise. But, you should always keep in mind that if you've been successful in securing an interview then the potential employer sees some potential in your qualifications and previous experience - even if it's not an exact match for the job requirements.
If the interviewer expresses concern about your lack of specific experience in selling complex software, agree with them that this is something that has never been part of your job duties before. Then discuss the skills that you do have and the experience you've gained in your previous positions that you expect to transfer over to selling complex software solutions. Also give a few specific examples of instances in which you have previously had the opportunity to take on new responsibilities with which you had no previous experience, and how your ability to learn quickly was helpful in your success in those areas. One or two little anecdotes about this type of past success will be helpful, so you should think back through your previous job history for a few good stories, in preparation for this interview.
You should also make it clear that, while you think your problem-solving skills and ability to learn quickly will be great assets, you do not presume to know everything you should ideally know about selling complex software solutions, and look forward to learning from your supervisor(s) and coworkers and calling on them as resources while you "learn the ropes." This is a good transition point for you to start asking a few questions about what sort of orientation or training will be available to you, if any, and how much supervision and support you can expect in this position, should you be the successful candidate.
You may also ask whether the interviewer feels that a lack of experience in this particular area is of major concern. Chances are that the interviewer will indicate that it is just one of many factors. But, if the interviewer does seem to focus on this particular weakness, you should reiterate the ways in which you are well-suited to the job, and stress previous success with situations requiring complex solutions. Again, having a few short stories in mind to relate about similar situations in your previous job(s) will be helpful.
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