An upgrade workflow is already ongoing on the SDDC manager.
The creation of a workload domain creation is required before the upgrade task is completed.
VMware SDDC manager 4.x
The option to launch a parallel workflow for a workload domain creation while already an upgrade workflow is still running is not available in SDDC manager 4.x.
A service lock enabled will prevent any other parallel operation.
This has been addressed from VCF 5.1.1 and later versions where the workflows are not bound to a service but instead in a workload domain level.