The property said that the nature of the living room cannot be changed and decoration is not allowed. Is this legal?

The specific way of house decoration does not belong to the scope of law. If the property thinks that your decoration violates the nature of the living room, this may be the internal regulations of the property, and you can communicate on your own or ask it to issue relevant evidence. If the other party can't provide it, you can decorate it in the way you want.