I must have missed your post somehow because this is very straight forward. I tried it with 4kvideo and 4k video and got the same result both times: "Operation failed. Some items are locked or require permissions."
That surprises me, but at least we've now covered all bases. Since you've tried and failed as root, I guess the file is locked.
Have you tried "Retry?"
Have you done a "Get Info" on the file to see if you can unlock it?
(See
https://www.finetunedmac.com/forums/ubbthreads.php?ubb=showflat&Number=63648#Post63648)