Hi, I'm playing in a campaign with a paladin and a ranger, and their cantrips dont show up anywhere on the character sheets, not even under spells. When you look at the spell's filters, there's only "1st level" and "concentrantion", but no "0". How can we fix this?

Paladin's and Ranger's don't get cantrips ordinarily.

You would have to get them through some other method like your Race or a Feat or one of the extended Fighting Styles etc..

The cantrips come from the ravnica backgrounds! We're now wondering if they should have gotten them lol. But dnd beyond lets them prepare the cantrips in "manage spells", but they dont appear on the spells tab

Those backgrounds say:

For you, the spells on the [Background Name] Spells table are added to the spell list of your spellcasting class.

So it's adding it to the class spell list - but if your class doesn't have any mechanic to learn the cantrip - then you can't use them.

I'm guessing if DNDBeyond lets you manage them - then that's a limitation - or a bug - with the system.

I'm testing this, and the background cantrips show up under Manage Spells for Paladin, but the Learn button is greyed-out. Which follows from Emmber's comment about classes that don't get cantrips.