"Don't have permissions" to view my backup files!

I backed up my old laptop using time machine as well as copied files directly onto external hard drive just in case... I wiped and traded in this laptop for a macbook air (OS sierra). When trying to access these files to transfer them, I am unable to open most of these files because it gives me the warning: "You don't have permission to view its contents."

I have tried to click on "Get info" and add myself under "Sharing & Permissions," but I am UNABLE to add myself, all it shows is "fetching":

This is very frustrating, how can I not have permission to see my own files?? Please help!

Would appreciate any advice! Thanks!!!

PS: wasn't sure which forum to post in, hope this is the relevant one.