Empty Trash Item in Use
I got a similar problem. My locked files were located in the trash bin of my Time Machine disk.

In the end, I forced the deletion by using the rm command.


Open your Trash window to actually see the locked files.
Open a terminal session to type the following (have your password ready)

sudo rm -R [drag and drop the locked files here]

Type your password when asked to do so.


Source: 
https://www.macworld.co.uk/how-to/mac/how-empty-trash-on-mac-3682422/



On a side note, I found something interesting: I noticed that the folder I wanted to remove showed as "Username's Macbook" in the Finder window, but in the terminal, it was like "Username/1/2/4/s Macbook". 

Too bad I didn't write it down. So I wonder if there was something wrong with the character encoding.