Never mind found solution in this thread here: https://dietpi.com/forum/t/emby-server/3083/1
Modified the following file at line 5224:
sudo nano /DietPi/dietpi/dietpi-software
Added the line:
local fallback_url=“https://github.com/MediaBrowser/Emby.Releases/releases/download/4.0.2.0/emby-server-deb_4.0.2.0_$arch.deb”
Seems to install now.