Dorico 4 can't find Vienna Symphonic Synchron plugin

I’m not a Cubase user but the Cubase forum does seem to have a few threads suggesting that M1 Macs are running Cubase 12 under Rosetta (Kind: Intel, shown in Activity Monitor) by default. See Cubase 12 in Rosetta only, even though Rosetta is switched off for instance. If you run Cubase 12 and take a look at Activity Monitor, does it show that Cubase is running as Kind: Intel?

If so, there’s your answer. Both Cubase 12 and Dorico 4 are universal - they can run natively on Apple silicon or under Rosetta - but in order to run non-native plugins they need to be running under Rosetta (and this is an Apple limitation rather than a Steinberg limitation - you’d find the same with Logic Pro or Pro Tools or any other DAW).