The minimum purchase at the ATM is $20 ($5 for Litecoin). Because of this, any bills inserted under a $20 bill ($5 for Litecoin) as the first bill will be rejected. However, after the first bill of $20 is inserted, the ATM will then accept bills of any denomination. If you are using a 20 as your first bill & it is still being rejected, please Contact Support  to let us know, as we may need to send a technician out to service the bill acceptor.