Why my battery health beta is not functioning?

I have been using iphone 6s for more than two years , when apple released new Battery Health Beta feature in iPhone i did not know much about it. Recently i came to know about this feature from a casual talking with my friends, so i tried to check it out. unfortunatley it did not work on my iphone 6s. I tried restoring via itunes but my try was in vain. So please guys can anyone help me with this error?… I am also attaching the error screenshot which i am seeing whenever i am opening battery health beta option in settings. i hope you guys will figure it out for me ….