Massively converting nitrogen gas to ammonia is a key process in modern agricultural and industrial fields. The conventional Haber-Bosch process for NH3 production has to be carried out under extreme conditions, leading to high energy consumption and huge emission of greenhouse gases. Electrochemical N2 reduction is a promising way for NH3 production due to its sustainable process and feasibility in an ambient environment. In this work, we screen the transition metals (TM), including 26 elements, supported on two-dimensional (2D) Nb2CN2 (TM-Nb2CN2) for their applications in the electrochemical reduction of N2 (NRR) based on first-principles calculations. We show that most SACs can bind with Nb2CN2 strongly through a TM-3N configuration. We find that Mn-Nb2CN2 is a promising candidate for the N2 reduction reaction (NRR), with a low overpotential of 0.51 V through the distal mechanism. Importantly, TM-Nb2CN2 presents high selectivity to NRR by blocking the hydrogen adsorption and preventing the hydrogen evolution reaction. Moreover, the scaling relationship and Bader charge analysis provide an insightful understanding of the mechanism for NRR on single-atom catalysts (SACs) anchored on 2D MXenes. Our findings may guide the design of novel substrates for SACs with effectively improved performance.