I only used ports as ScanClock. But for scan shift/scan output stuff, I used internal signals, because I did not manually connect them to ports. I left the job to stitching tool to connect them.
set_dft_signal -view spec -type ScanDataOut -port ioxxx -hookup_pin "uxxx/xxx"
So it seems that you need to use "spec" instead of "existing" and use "hookup_pin" opt.
But you should be able to manually connect, since dft can only control ports... the scan clock should be eventually from outside of the chip