When you claimed the device/serial/order number it would have most likely have claimed the license associated to the device. Assuming the customer has a different organization to your current one, you'd need to progress with a "license transfer" to transfer the license from your current organization to theirs. Have a read of this article which will shed some more light and provide you with some good instructions on what to do - https://documentation.meraki.com/zGeneral_Administration/Licensing/Removing_and_Moving_Licenses
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Another thing that has made the easier for me when doing the license transfer. Make sure you yourself exist on both Organization Dashboards, as an Organizational Admin. When you contact support for the transfer, this will make it easy.
Also, the transfer of the license could have an unforeseen impact to your license co-termination date. Also it is possible for the license itself on the MX64 to be expired but with the averaging of other licenses your organization is sill licensed.
The device was originally in your organisation and you have the licence. You unclaimed the MX64 hardware and moved that hardware to a new organisation. That device is now sitting in your customers organisation without a licence.
You could try transferring one of your licences to your customer and back again - but I wouldn't do that. A compromise if you want to continue down this path is to create an extra network in your organisation and put the MX their. Then grant your customer access to that network. That will solve the licence issue and let the customer try out the MX.
But this is hard because you aren't using the approach that Meraki is recommending for trials. What you can do is use the Meraki Try and Buy program for your customer. Then everything will be easy for you and your customer.