Thanks Chris,
It works now.
I used the information you provide at your link.

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To resolve the issue you have to:

1. Ftp to your server;
2. Go to Includes subdirectory;
3. Open a file called upgrade_listings.inc.php; and
4. Add a string:

$rlHook -> load('phpListingsUpgradePlanInfo');

Under the following code:
/* get plan info */
$plan_info = $plans[$plan_id];
$current_plan_info = $plans[$listing['Plan_ID']];

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