[Fenris447]

You should start with 3 cantrips as a basic cleric. Once you've selected the Death Domain, you'll see a dropdown for Reaper in the Class tab to select an appropriate necromancy cantrip. Then, once you've selected that cantrip there, go to the spells tab and add it as an additional cantrip to your list (even if it says "no more cantrips allowed"). You should then have a total of 4 cantrips and no errors.