Dani, while now isn't the best time to prop succulents, it can be done. Bottom heat (the kind you'd get from sunlit concrete, for example, and your plant on the concrete) would help root a cutting, but you can go one better by getting a seedling heating mat and simulate that. But I think maybe the problem too is that the cutting needs some sun to help it root. It also DOES NOT need watering at all, as it has no roots. The best you can do at this time, IMO, is spritzing the cutting with a spray bottle every few days. Also, set the dusted cutting aside for a few days before planting - this allows it to callus over. And the callused cutting should only be slightly inserted into the potting medium.

Is your plant Euphorbia ingens?

hi, thanks for your advice!

it is a euphorbia ingens. my issue is with the cutting; it won't callus without rotting/molding. do you have any insight on best callus-ing environment?

A room where there's some air movement - leave it out of soil for a few days after cutting and prepping it for planting, allowing the cutting to dry, too.

Warmth is a prerequisite for best success in rooting Euphorbia from cuttings. You could try taking a cutting and placing it on top of the fridge or atop a fluorescent light fixture for a couple days before attempting to root it.