Yolov4 darknet training - Couldn't open file: data/train.txt

I've been trying to train a custom dataset but I'm getting the following error:

Reading files:

with open('data/obj.data', 'w') as out:
  out.write('classes = 3\n')
  out.write('valid = data/valid.txt\n')
  out.write('names = data/obj.names\n')
  out.write('backup = backup/')
  out.write('train = data/train.txt\n')

So when I run this:

!./darknet detector train data/obj.data cfg/custom-yolov4-detector.cfg yolov4.conv.137 -dont_show

The first half of result seems fine, but then I got the following messages:

Done! Loaded 137 layers from weights-file 
Learning Rate: 0.001, Momentum: 0.949, Decay: 0.0005
Couldn't open file: data/train.txt

It seems so train.txt file couldn't be found for some strange reason