@Lucilla21 Unfortunately you have started hosting at a time when Airbnb customer service is abysmal (they laid off a couple thousand staff or more, due to reduced income due to COVID) and both hosts and guests have had weeks long waits to get assistance. But other hosts here are often more helpful and knowledgable than CS staff, so you came to the right place.
You'd have to post your actual nightly price, the amount of the monthly price, and what you actually received, in order for other hosts to be able to help you figure out what might account for the missing $150.
If it's any small consolation, you wouldn't be the first new host to find themselves hosting for practically free because of not having your pricing set correctly to what you wanted. Discounts are cumulative, so if you have a new listing discount on and also a 45% discount on a monthly stay, that means guests will get 65% off what you would normally want to charge. You'll just have to take this one as a learning experience (there are many) and change your pricing ASAP so it doesn't happen again.
One thing to be aware of is that experienced hosts don't listen to Airbnb's pricing suggestions. Those have nothing to do with them wanting you to be successful, they are only for the purpose of getting hosts to drop their prices to absurd levels so that guests will book (a 45% monthly discount seems way too high to me). Which means Airbnb gets its service fees, the faster the better, as far as they're concerned. Whether that means you can barely pay for the expenses associated with hosting isn't of any concern to them.
Decide for yourself what you are willing to rent for (don't sell yourself short), based on your own expenses, how much you need to profit to make hosting worthwhile for you, and what the market in your area will bear, and don't pay attention to Airbnb's pricing suggestions.
It's also not a good idea for a new host to take long-term bookings, because shorter bookings means you'll get reviews built up quickly. (Long term bookings aren't advisable for a number of other important reasons, too, and there are threads about that here on the forum- do a search). As a new listing, you get a boost in search rankings anyway, so that 20% off the first 3 bookings, I would shut off. That 20% off can help to get bookings quickly to build up reviews, but you've already negated the point of it by taking a long term booking.
Don't fret about about your newbie errors too much, just get your pricing right and move on. And post the actual numbers here if you'd like other hosts to try to explain what happened with that missing $150. (it may, in fact, be an Airbnb error)