The error "Citrix Licensing Manager could not complete the operation. If you are trying to allocate a license, go to My Account. For assistance, contact Citrix Customer Care" is returned after allocating a license using Citrix Licensing Manger.

The error message indicates that Citrix Licensing Manager is not able to contact citrixservices.citrix.com. This can happen due to multiple reasons, including there was simply no connectivity from the License Server to citrixservices.citrix.com, or the presence of a Firewall or Network Proxy in the environment. 

To troubleshoot the underlying cause of the failure, please follow the steps below:

1. Make sure the License Server has network connectivity and can connect to citrixservices.citrix.com over port 443
2. If you will have to configure a proxy, please refer to the following documentation for the steps: Configure a Proxy server. Please note that Licensing service needs to be restarted for the changes to take effect.
3. If there are no proxies required to reach citrixservices.citrix.com from the server, it is possible that there was a network error at the time. Try to complete the action at a later point. 
4. Alternatively, use the Manage Licenses portal to allocate and install the license, following the manual steps outlined here
5. If the steps above doensn't resolve the issue, there might be an issue with the code you were assigned; contact Citrix Customer Care