Check GateKeeper ...
Open Sysem Preferences > Security & Privacy then select the General tab.
Select: Mac App Store and identified developers
If that area is greyed out, click the padlock icon to make changes. You may be prompted for your admin password.
OS X: About Gatekeeper
If that didn't help, disable anti virus software if installed. Then try logging in.
And try deleting the cache associated with the App Store.
Quit the App Store if it's open.
Open a Finder window. From the Finder menu bar click Go > Go to Folder
Type or copy paste the following:
~/Library/Caches/com.apple.appstore/Cache.db
Click Go then move the Cache.db file to the Trash.
Relaunch the App Store and try logging in.