same problem with mojave 
here you can find old releases:
https://download.nextcloud.com/desktop/releases/Mac/Installer/
and here the 3.10.0 (latest version without macos 12 requirement:
https://download.nextcloud.com/desktop/releases/Mac/Installer/Nextcloud-3.10.0.pkg
I hope Nextcloud is going to support Mojave again in 3.10.2.