I looked on this website RoonOnNAS – User your NAS as a Roon Core to find a way to install roon on my QNAP. But i couldnt find the app in the app store, i have a ts-421 with 4 bays and the newest version 126.96.36.1994. Am I missing something?
I just checked your qnap model. Roon Server will only be available on model with x86-64 cpu architecture. It seems the TS-421 is using an ARM cpu (Marvell) according to its technical specs.
I am sorry, but installing Roon Server on your QNAP is not possible.