A sale fare shouldn't preclude you from upgrading the domestic leg, there must be another reason as to why it isn't allowing you to submit the upgrade request online.
This has been discussed on here previously - https://www.australianfrequentflyer.com.au/community/threads/beware-of-dcc-in-europe.95324/
Dynamic Currency Conversion (DCC) often gives a less favourable outcome than paying in the local currency.