Are you talking about at the URL opera://discards?
Since Opera GX is based Chromium 135 now, some changes in Chromium with discards might not be compatible or broken with Opera. Or, Chromium 135 might have some bugs with discards that's fixed in newer versions of Chromium (Chrome Stable is on 138). Not sure.
Testing in Opera GX 120.0.5543.66 64-bit on Windows 11 23H2, clicking "Urgent Discard" for a tab for example doesn't do anything for me.