WebCore is A new version of core, but since it’s still early development you might want to look at Core instead, as it should also be able to handle luminance triggers.
I’d also definitely get in touch with smartrules support through their website. The developer is very active in this forum and i’m sure he’d want to know about any problem you’ve run into.
Finally, any reason you’re not just using the official smartlighting feature? It should be the simplest of all of these. There’s a trigger for “illuminance” and then you can choose the device that is monitoring lux levels and The specific level to trigger at. This feature was added in a later update so it’s not listed in the official knowledgebase article, but it’s definitely there.
https://support.smartthings.com/hc/en-us/articles/205380404-Smart-Lights
The only thing with that one is instead of changing the Mode directly, you’d have to turn on a virtual switch and then have a routine that changes the mode when that virtual switch goes on. So now that I write that out, that’s not the simplest option for your specific use, The third-party smartrules app would be. The official lighting feature would be the simplest if you just wanted to have a light come on based on luminance.
Welcome to SmartThings, where there are typically four or five different ways to do something, none of them intuitive. 