A landlord can ask you to pay rent before the first, but you don’t have to agree to that. A landlord can ask anything they want, but you only have to do what you’ve agreed to in the rental agreement. If you’ve been in the rental unit for a while and the rental agreement states a due date on or after the first of the month, the landlord cannot unilaterally change it without your agreement.
If this is a new rental for you and you haven’t yet signed a rental agreement, explain to the landlord that you can’t pay rent before the first because that’s when you get paid, or whatever your reason for wanting to pay the first of the month might be. If you have a compelling reason why you want to pay on the first or after, the landlord should agree to that. If not, there are a lot of good rentals out there that have more reasonable landlords.
Landlords tend to like to negotiate, but remember that you’re proposing to fill an otherwise vacant rental for them. There is value in that. Don’t let the landlord fool you. Filling that vacant rental is, or should be, more important to the landlord than having his/her way about when the rent will be due. If not, and the landlord wants to be unreasonable about it, you may want to reconsider your choice of a rental unit. How would it be to have to deal with an unreasonable landlord the whole time you’re living there?