Description: This tool allows an administrator to recover access to a file that was denied by re-assigning file ownership.
takeown /f //HKEY_LOCAL_MACHINE/MyKey/ \R \A
You're saying take ownership of file located at network location HKEY_LOCAL_MACHINE with share name of MyKey, which is not the way registry keys are stored.
Also, \R and \A should be specified with forward slashes, not backslashes, ie, /R /A
To take ownership of registry key,
Open Registry Editor.
Click the key you want to take ownership of.
On the Edit menu, click Permissions.
Click Advanced, and then click the Owner tab.
Under Change owner to, click the new owner, and then click OK.
To do so via an external program you'll need an App like RegDACL
What is RegDACL good for? RegDACL allows you to query and change the discretionary access control list (DACL) of any Windows NT Registry key. You can of course use the program RegEdt32 to set registry permissions, but if you need to edit more than a few machines then you are faced with quite a boring job, not to mention being more prone to make mistakes. RegDACL gives you the ability to use a batch script to perform this job. The freeware versions of RegDACL allow you to define access permissions for the predefined groups"