@aarm A few suggestions

• Does the VM have any snapshots, which could be blocking the disk removal? If so can you remove them?
• Are you trying to remove the disk whilst the VM is powered off or on? If on, have you tried when powered off?
• Try vMotion to another host then try removing the disk
• Try Storage vMotioning to another datastore then try removing the disk?

One of these may clear the lock or whatever is generating the error.

HTH