From the 7.6 release notes:
When you use Sysprep to customize Windows 10 linked clones on vCenter Server 6.7, the linked-clone desktops get stuck indefinitely in the customization state during provisioning or recomposing operations.
Workaround: Use vCenter Server 6.5 U2 or earlier. If you must use vCenter Server 6.7, then use the Quickprep customization method.