In this research, we investigate the nonlinear energy transmission phenomenon in a metastable modular metastructure. Numerical studies on a 1D metastable chain demonstrates that when driving frequency is within the stopband of the periodic structure, there exists a threshold for the input amplitude, beyond which sudden increase in the energy transmission can be observed. This onset of transmission is due to nonlinear instability and is known as supratransmission. We show that due to spatial asymmetry of strategically configured constituents, such transmission thresholds could shift considerably when structure is excited from different ends and therefore enabling the non-reciprocal energy transmission. We demonstrate that the critical threshold amplitude can be predicted analytically using a localized nonlinear-linear model combining harmonic balancing and transfer matrix analyses. These investigations elucidate the rich and intricate dynamics achievable by nonlinearity, asymmetry, and metastability, and provide new insights and opportunities to accomplish non-reciprocal wave energy transmissions.