Can't open a .nc file

[huddao]

f = Nio.open_file(ent_dir+'/merged/'+j+'.nc', 'c')
File "/public/home/soft/anaconda3/envs/meic2wrf/lib/python3.8/site-packages/PyNIO/Nio.py", line751, in open_file
file = _nio.open_file(filename,mode,options,history,format)
Nio.NIOError: Unable to open file

The .nc file is exist.