It’s an Access Denied error. Is the account that’s running the command also an administrator on the target computer? If not, you’ll also need to use the -LocalCredential parameter in addition to -UnjoinDomainCredential. (You can pass the same credentials to both parameters, if appropriate.)
Which account? There are two sets of credentials in play, in the original command you posted. C_NET\Administrateur is used to connect to the domain controller and perform that part of the unjoin process. To connect to the VSMW0020 computer, though, the cmdlet is just using the credentials of whatever user is logged on and running the command. To use the C_NET\Administrateur account for both parts of the operation, you’d do something like this: