Roundcube Mailbox doesn't exist: Trash

Have a look in /var/lib/roundcube/config/defaults.inc.php (in Ubuntu 16.04.1) if the following lines exist and are correct

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// store draft message is this mailbox
// leave blank if draft messages should not be stored
// NOTE: Use folder names with namespace prefix (INBOX. on Courier-IMAP)
$config['drafts_mbox'] = 'Drafts';

// store spam messages in this mailbox
// NOTE: Use folder names with namespace prefix (INBOX. on Courier-IMAP)
$config['junk_mbox'] = 'Junk';

// store sent message is this mailbox
// leave blank if sent messages should not be stored
// NOTE: Use folder names with namespace prefix (INBOX. on Courier-IMAP)
$config['sent_mbox'] = 'Sent';

// move messages to this folder when deleting them
// leave blank if they should be deleted directly
// NOTE: Use folder names with namespace prefix (INBOX. on Courier-IMAP)
$config['trash_mbox'] = 'Trash';

// automatically create the above listed default folders on first login
$config[‘create_default_folders’] = true;
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The restart apache2 and reload page.