Failed to detect objects without any error

[kalikhademi]

I am training the detector on my dataset to detect one specific object ( number of classes is equal to 1). I use the yolo3.weights from original website. I do not recieve any error but when I run the command:
./darknet detector batch build/darknet/x64/data/obj.data cfg/yolo-obj.cfg weights/yolov3.weights io_folder sample_imgs/ output/ -out output/result.json -ext_output > output/result.txt

the result.json objects are empty and there are no bounding boxes drawn on the images. For each picture in the sample_imgs folder I recieve this log:
Not compiled with OpenCV, saving to output/18T_1076301.png instead

Also during the detection, I recieve the following warnings:
Cannot load image "data/labels/126_7.png"
STB Reason: can't fopen
How can I change it?

Thanks,
Kiana