Sorry, but there is no immediate recourse. A landlord is "allowed to discriminate" when it comes to credit score or criminal history. This means even if you have a criminal history, the landlord can decide to reject the application. And/or, if your credit score is considered low, the landlord can reject it as well (the landlord can set their own minimum requirements for credit scores for applicants).
There is no "right" to rent. As long as the landlord is not discriminating due to race, religion, creed, etc, then they can reject an application based on credit score or criminal history, whatever it is.
Other landlords may not be so picky - this one may have simply had very high standards. But there is no way to "ensure" that a rental application would not be rejected in the end, if the landlord has certain criteria.
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