I was trying to use maximum likelihood in SCP QGIS, but a warning come out "Information : Select at least one signature", and when I selected all signature, it still won't run.
After reading here and talk with the developer, finally, managed to resolve this.
It is because I was using drone imagery with a normal and IR camera as sensors. With this kind of camera, there is this alpha band which have all same value. When I remove alpha band from raster file, leaving RGB only, the problem is resolved.