You want to extend a VMFS datastore using vCenter, but the available unused space on the datastore is not listed in the wizzard?
The reason for this is that vCenter Server applies filters to prevent possible data corruption on LUNs used by other hosts. If you connect directly to the ESXi host you can extend the datastore as the filters do not exist there:
So if you are sure that you want to extend the datastore, connect to the ESXi host directly and extend the datastore from the GUI. Do not forget to rescan all hosts from vCenter Server afterwards.
You can learn more about storage filters here: VMware DOCs
Please note: it is not recommended to turn off device filters!