Letting someone think something is a tough area to deal with. You have to evaluate the obligation you have to correct the error. You also have to evaluate how much responsibility you have for the error. If you say positively you know something when you don’t, you’re responsible for the resulting misimpression, and I think you put yourself in a bind because if you do get the job, IMHO, you are obligated to correct that misimpression, and that’s not a good way to start a job.
But if you simply omitted information, and didn’t actively mislead, your responsibility has diminished, and you may not have any obligation to make a correction. People rarely state the things they don’t know, and it is the responsibility of the employer to find out if you meet the requirements. If you’ve honestly listed your skills, and they continue to discuss employment with you, they might realize you didn’t meet some stated qualification, but may not care. And if they did care, they should certainly have asked you directly.
So my advice is don’t say something that isn’t true, and don’t list something that might be a disqualification, and in the end the missing proficiency in Excel and Access may not matter at all.
ETA: Beaten to the punch. And with better material advice instead of unnecessary deconstruction of the problem.