Why ffmpeg didn't respond to custom name as output?

Working:

import ffmpegimport ospath = "K://Delete//"dir_list = os.listdir(path)for i in dir_list:        ffmpeg.input(path+i).output("K://t//new%d.png", vf='fps=1/10').run()        os.remove(path+i)

Not working:

import ffmpegimport osfrom datetime import datetimepath = "K://Delete//"dir_list = os.listdir(path)for i in dir_list:        dt_string=datetime.now().strftime("%d%m%Y%H%M%S%f")        ffmpeg.input(path+i).output("K://t//"+i+"%s.png"%dt_string, vf='fps=1/10').run()           *#%d or %s both not working.*        os.remove(path+i)

I tried to avoid same name files to be generated. Because that will raise error in future.

I tried to give all the files unique names that is appending datetime as a String to the file.

But ffmpeg is showing error that "Conversion Failed!"

Why is this happening and how can I fix it?