mAP nearly zero?

What I'm trying to do

I tried to train on the VOC2007 dataset. However, I failed. After many epochs, the mAP is zero.

What I've tried

1. Loaded with DarkNet weights——weights/darknet53.conv.74
2. Converting VOC datasets to COCO： filename.xml -> filename.txt x = (x1 + x2) / 2 y = (y1 + y2) / 2 w = x2 - x1 h = y2 - y1 x , y, w, h = x/W, y/H, w/W, h/H
3. modified the yolov3.cfg: Use the default hyperparameters Use the anchor by kmeans Change the class from 80 to 20 Changing the feature map dimension from 3*(5+80) to 3*(5+20)

Additional context

I tried to modify the learning rate from 0.0001 to 0.001， did not work. I tried to use sgd but not adam, did not work.