We’ve been granted access to our client’s Piwik account, but we’re encountering difficulty in accessing the analytics. We are consistently receiving a notification stating: “No site or app selected. Choose one from the picker in the upper right corner.” Could you please advise why this issue is occurring and how we might resolve it?
Thank you for your assistance.
Best regards,
Hi @RubikonGral, Welcome 
This message suggests that when you log in to your account you need to pick the Site or App you want to see the data for (make sure when you log in you are in Analytics - click Menu-Analytics).
Look at the top-right of your screen and find there a drop-down. When expanded you will be able to click the site name.
It may happen there will be no sites listed, then you need to get back to the person who created your account and ask them to grant you permissions for that site or app.
Tymek