@Maya388
You can ask for more money via the request money feature in airbnb, but in all likelhood this will result in a negative review from the guest which you probably do not want this early in your hosting career. If it's not a lot of money, I would suggest letting it go this time around, espeically as it doesn't sound like the guest was trying to be deceitful, it sounds like a genuine misunderstanding. In all liklihood the guest saw that your room sleeps 2 and thought the price was for 2 guests. A lot of guests don't realise that they need to enter the number of guests staying and that this can influence the price. Some listings charge a flat rate and some charge per guest, so it's understandable that guests can get genuinely confused.
You could probalby be a bit clearer about this in your listing and/or your house rules.
You could say something along the lines of "only guests booked and paid for on the platform may stay at the property, including visitors. This is an insurance requirement. If you have booked for one and have another guest staying you will need to amend the reservation"
Also when someone books it's a good idea to clarify how many people are staying - eg "I see you have booked for one and i'm just double checking that this is correct as only guests booked and paid for will be able to gain access to the listing"
OR you could just make your base rate for two people not one, to avoid this problem ( but this may make it less attractive for solo travellers)
It's always helpful to be very clear about your expectations in your listing. guests may not read it, but if you have it stated you are better placed if a disagreement arises.
Regards