Hi, OP! So firstly, I need to ask: •How long ago did you make this arrangement? •Did you receive the succulents bare-rooted on the mail, or did you purchase them potted from a local plant store then repotted them at home?

Most of my succulents (which spend between 5 days to 2 weeks being airdried and shipped) usually arrived with dry, brown roots. That means, when I potted them, they had to start growing new roots all over again. While actively rooting, it’s actually pretty common for the lower leaves of succulents to turn yellow and slowly dry up. This will stop once they stabilize.

With that said, I think the bigger issue here is root aeration and soil drainage. One, that looks like a glazed ceramic pot which has a really high water retention property. And two, those pebble toppings are too much for (what I assume are) newly-potted succulents.

You will need to remove the toppings for now to allow the roots to breathe. You may choose to keep the ceramic pot but make sure you are using a 70-30 soil mix that drains and dries fast (70% gritty materials such as pumice, perlite, vermiculite, lava rocks, etc and 30% organic components such as cocopeat, vermicast, etc).

I also suggest setting up a fan if you don’t already have one. My succulent casualties went down by about 75% after I installed a mini fan for my indoor growing space.