Unexpected error in data traning

[sunqiangzai]

I am a newer for machine learning. I got the following error:

here is the code: python train.py --dataset test --arch UNext --name test --img_ext .jpg --mask_ext .png --lr 0.0001 --epochs 500 --input_w 512 --input_h 512 --b 8

and test data folder structure:

How can I fix it.