Hey - Because this can be caused by one of a number of issues, the exact course you are to take to correct the condition will depend largely on the exact response (or error message you receive) when attempting to install an app. We will be able to best assist with this piece of info if you can comment with it below. Do try, otherwise, to wipe Google Play Store's data and retry - Settings > Apps > All tab > Play Store > Clear data ... confirm and restart the phone.