OK.. life is never meant to be easy.
I suspect your USB-C drive with whatever adapters you are using to connect to the TC is simply not compatible with USB2 on the TC. That leaves you in a bit of difficult place. You cannot copy with Finder and you cannot Archive from the TC.
What are you actually trying to achieve? Were you hoping to continue using the existing backup? That is not possible. You really have to start over. If all you are doing is really to archive the old backup let me suggest you use a backup software.
Carbon Copy Cloner. It is free to download and use for a month. Set it up to copy the Backupbundle from the TC to the USB drive.. it will be much more likely to work because it uses rsync which is decent command line tool for most Nix distributions.
Once copied to the USB the backup should not be used by the computer.. indeed you probably cannot make it work anyway but our experience is backups do not move well. And the computer backs up differently to local drive cf network drive.
An alternative would be to buy another cheap 3 or 4TB portable drive and plug that into the TC and it should be recognised and you can then create an archive.. that allows you then to wipe the TC and sell it off.. After a few months of backup to the new drive you can wipe the archive if you like and use it for something else.
