You’ve mentioned the “theme” but I’m talking about plugins which are usually separate to a theme. Plugins are usually stored in the /wp-content/plugins/” directory. So if you had a plugin installed called “MyPlugin” I would normally expect to see a folder in there like this: /wp-content/plugins/MyPlugin and if I didn’t want that in there I would just delete that “MyPlugin” folder and it would be gone. That file path in the error is not a “page” you can link to in a browser, it’s showing you where your current installation of layer slider apparently is.
Here is the WordPress information on managing plugins:
If you are familiar with using ftp, you would usually be able to connect to the site, open that folder and see if there is a LayerSlider folder in there and delete it. This would remove it from your WordPress install. I wouldn’t do that if it was working fine and I wanted to re-activate it at another time or if I wanted to save any associated data. I’m only suggesting deleting the folder because it seems like you already have an installation that you are unable to activate that is causing problems. So deleting it and trying again would be the first thing I personally would try to do.
I think that perhaps you really need need to go to the LayerSlider support (if they have it) for these questions though if that doesn’t fix the problem.