Currently, terrain correction of ALOS Level 1.5 are not supported in SNAP. But there is a work around which at least allows geocoding: ALOS-1 L1.5 Terrain Correction
Currently, terrain correction of ALOS Level 1.5 are not supported in SNAP. But there is a work around which at least allows geocoding: ALOS-1 L1.5 Terrain Correction