I added this mod to a saved game and enabled the more intermediate option.
The recipe solar cell is not enabled. The solar energy technology is already researched.

What am I doing wrong?

If I understand correctly, it sounds like you researched solar, then added this mod later. That needs to go the other way around.
Recipes unlock when you complete research, so recipes you add after won't see the trigger and will never unlock.

You can either look up a console command to unlock the recipe or restart the map.

"Recipes unlock when you complete research" /sign.
Mods can have functionality, when the mod is added later to a game, to check if tech was already researched and then enable the recipe.
Its described here https://forums.factorio.com/viewtopic.php?t=23837

/sign? I don't understand.
I hope that's not a typo for '/sigh' because if you're linking forum threads at me while sighing you already know what you did wrong and don't need to ask.
It would, however, turn this from a "Why isn't this working?" to a "Please add this functionality" post, which is not unreasonable.

/sign is for I agree with you.

I do not know if its a bug or not. It might be, that the author of the mod has implemented that the recipe is enabled when the tech was already researched.
If it is not implemented then this post is a feature request to enable the recipe when the technology was already researched.

okay, thank you for clarifying. I was getting defensive over nothing.