When I download a free app it says"action:authenticate:sm"

Hi,

I too got the same problem.
If you have not signed out, then signout. Then, after restarting my iPhone, again I signed in. Problem is solved.

I Also got this problem but I just can't sign into iTunes overall. I have an iPad Air running iOS 9 and have been having issues with the amd action message recently and nothing has worked, nor does your solution... I am getting very frustrated as I cannot download any apps or update anything... I can't even sign into Apple Music like solutions say because it just does nothing, no error box, nothing....

Thanks. This solved my problem.

Just adjust the mac or iPhone clock to your current location ... that's it ... simple yes, but it should fix your problem immediately ...