Hello Mate,
I had a same issue few month ago, in which I had to move a VM of about 10TB.
Once I initiated the storage vmotion, it showed some behavior but didn't start in actual. I was not having any datastore which is more than 10TB rather I had 5x5TB datastores to move VM files.
Either you can move the VM to a datastore which have free space greater than your VM size, or simply shut the machine off and move the HDDs one by one.