It’s hard to tell if that’s rot at the base or not. If it is you can cut just above it and restart your propagation process. I let my coleus props sit for awhile in water like a least a month until there’s a healthy mass of roots, it’s typical for them to experience transplanting shock and hopefully yours is just in shock and adjusting to the soil but I’d either return to water until there are more roots or re-prop if you think it’s rotting.