Can not install/download APPS via Play Store to my Tablet ....how can i fix this? Over the past 2 days, whenever I try to download/install an APP via the Play Store/Market app on my TABLET, the app thinks for a while and then displays this error message:

Authentication is required. You need to sign into your Google Account.

But I am indeed signed into my Google account, and as seen if I open the menu in the Market app and click the "Accounts" button.

Does anyone know how I can solve this issue? 

thank you in advance!
I had to re-authenticate on my tablet, too. I assume Google is trying to detect fake accounts used to push fake-apps into the top apps list.

May be in your case there is a conflict between the cached data and the actual store data. 

You can try to clear the app cache of "Google Play-Store services" app and the Google Play-Store app. 

Additionally you should check your Google account in "Settings" -> "Accounts". Manually perform a synchronization.