Steven, do you mean your listing needs 100 pounds security deposit as part of a new booking (if so you can easily set this up in your pricing section). Or is your question regarding making a claim? If it is regarding making a claim then you need to go to the resolution centre or find your reservation and click on "report a problem" and go through the steps. Please keep in mind that you need solid evidence to make a claim and the guest has to agree to it. Airbnb will only mediate on your behalf if the guest ignores the request and you have solid evidence to back the claim. A general request for money will most likely be ignored. Claims have a 48 hour window after departure, that is standard. In most legit cases you can make a claim, the system is not against you. As you have not been specific in the issue this is all i can advise you.