Trying to delete a folder on an external hard drive, but met with the message: "The operation can't be completed because backup files can't be modified"

We have shared an external hard drive as a family since 2016, however I am now going through the process of transferring all of my assets onto my own personal external hard drive. Therefore, I want to delete all of my assets on the shared hard drive as they've been moved over. However, If I go to right-click and 'Move to Bin' the folder 'Backup (Michael)', I get the error message "The operation can't be completed because backup files can't be modified".

I've therefore attempted to delete the folder in Terminal, however I am met with the message "zsh: unknown file attribute: i".

Surely there must be a way to delete folders on external hard drives? I have Googled these queries I have been presented with, and am very lost.