Actually you can not do that, its because they have locked the maximum shutter speed to 1/8. I tried finding solutions online and then also sent a mail to the developer team that was provided, didn't get a satisfactory reply from them. I asked them again but they never replied. They have clearly ripped off all of us who bought the device because it was marketed as a camera centric device, I'm not reccommending any Aseries device to any one who is planning to buy a samsung midranger. I myself use a Chinese phone now because of the same reason. They can actually fix it with a software update, but they are willingly not doing it.
you can do that with another way cos a50 cannot control the shutter speed or the aperture.you must use a tripod for your phone and take the same picture for 15 times or more or less but i think 15 times is ok.then you must compine tese photos with an apk .you create the same efect but i do not know if you can do that for light trails.for sea or water is ok .