if you've read my previous posts, I've written that I'm checking in
volumes with checklabel=barcode that means all volumes are barcoded and I
think that also means there is nothing to label. Or am I wrong?

The cartridges may well have barcode labels affixed to the outside; that
does not guarantee that the tapes are labelled internally to match.  The
error message you posted points to the possibility that there is no
internal label on the tapes.

If there is a question whether tapes are labelled, check them out of the
library and run the label libv process to attempt to write a label onto the
tape.  Activity log messages resulting from that process will shed light on
whether the labelling was successful.  Label libv (without "overwrite=yes"
will fail on tapes that are already labelled, or there could still be an
I/O error logged.

Ted