Hi, the "Guest Customization" starts couple minutes after the is powered-on, so make the VM is powered-on and allow some time.
You can monitor if the progress of the customization by selecting and under Task & Events and select Events and you should see the status the customization (see below screenshot). You will be able to track down if it has started, success and failure and so o...
I am assuming the OS is supported, that you're running VMware vCenter and the VM is powered down.
Within the vCenter Web Client, you can right click the VM, and select Guest OS > Customise Guest if it is just for a single machine that has been deployed without the Customise Guest option selected during the clone phase.
Thanks for your quick response,
I'll try to check and will get back with an update.