Why: There was an issue signing you into YouTube. Troubleshoot here.
Did you attempt to sign into YouTube with a Gmail account (after signing out with another Gmail account) that has not signed into YouTube before or for the first time? If so, you likely received the “There was an issue signing you into YouTube. Troubleshoot here.” message. Generally, refreshing the same login window will sign you into YouTube normally.
Otherwise, try deleting your browser cookies. If this does not work, sign in to YouTube using a different browser. Finally, ensure you are not logged into another YouTube or Gmail account in another browser window.
Learn more in this Google article.