This is the line with the error
sprintf( __( "You must have a <a target='_new' href='%s'>PayPal Sandbox</a> account setup before using this feature.", APP_TD ), 'http://developer.paypal.com/' ),
sprintf( __( 'You must have a <a target="_new" href="%s">PayPal Sandbox</a> account setup before using this feature.', APP_TD ), 'http://developer.paypal.com/' ),
Might be your usage of single/double quotes.