Replacing a switch with neutral to a Caseta switch without neutral

I've got a light-activated photocell switch that I'd like to replace with the standard PD-6WCL dimmer. The photocell uses a neutral wire, so it's got a white, red, and black wire. The Caseta switch does not. Here's a diagram I drew of the current (working) configuration: https://imgur.com/a/OCuB8qk The three things at the top are bundles of wires coming into the box. The three circles in the middle are wire nuts. So...no problem, right? The Caseta instructions seem pretty straightforward for this situation. You don't use the neutral, so I removed the existing switch from the white/neutral bundle and tightened that nut back up. I removed the two remaining connections from the existing switch and connected the Caseta to where they were connected. One line to the wire nut with the red wire, and the other one to the nut with the black wire. Powered back on and...nothing. The light will not light, and the green LEDs on the Caseta switch don't turn on. What am I doing wrong? Could I just have a bad dimmer? [edit] sorry guys, this forum removed all my line breaks and won't let me edit/add them back in.

IS the light truly dimmable? Was the photocell part of the switch or the light fixture. Typically Smart dimmers/switches do not work with photocell fixture as the photocell controls the lights turning off and not the dimmer/switch. What is the wattage of the light in the fixture? Does it work with an incandescent light?

The photocell currently *is* the switch in its own 1 gang box, and is not part of the fixtures. The lights that the photocell is controlling are two 12W dimmable recessed LEDs.

Thank you for the explanation. Sounds like a compatibility issue or a bad dimmer. A good test would be to add a incandescent bulb to the circuit. However that may not be possible pending the type of recessed LED's these are. You could always move the dimmer to another location to see if it works there as well. If both of these ideas do not work then it would point to the dimmer.

Three months later I solved this. Posting here in case anyone runs into the same (uncommon) issue. My wiring was correct. The light I was attempting to dim was a recessed can light. What I didn't know (and wouldn't have had any reason to look for) is that that recessed LED had an inline bulb dimmer installed up inside the housing. Something like one of these: https://amzn.to/3jsxVlv So really I was attempting to dim a 12w LED which was already being dimmed. There just wasn't enough power draw for the Caseta switch to function. I removed that screw-in dimmer and the switch immediately started working.