Can I run an electronic piano using a power bank?
Yes, it should work. The amperage of a power supply is the max current that can be drawn, but a load will only use however much current it needs. So having a 3A power supply powering a 1.5A load will work as long as the voltages are the same. The only thing left is to make sure the polarities are the same, since the piano takes a center positive barrel jack, the power bank is almost certainly the same but it's worth a quick check to be sure.