Applications where the APK is not published alongside the source code in GitHub or whatever, and is instead only available through an F-Droid repo (like Mull), take some extra configuration to set up through Obtainium. Fortunately, there is this neat crowdsourced list of popular apps that require that type of extra configuration, where you can find an app like Mull in the list and quickly import its configuration into Obtainium

https://apps.obtainium.imranr.dev/