I dont see them in the crafting menu. "Fluid handling" researched

You don't see them to the right of the regular tank, as depicted here?
Can you try summoning "6x6-underground-tank" from the console, that way I know if it's appearing in-game at all, then we can try to figure-out what's gone wrong with injecting the recipe unlock into that research technology.

Yes, to the right of the vanilla tank is empty. Using the console the items added, this worked for me.

Strange. I've verified things are working as they should on a fresh vanilla install of Factorio. I'm guessing we've got a mod conflict.
Can you post your mod list please.

I tested the mod on a new map and everything works as it should. It turns out the problem is to add storage to the map where the technology has already been researched.

I have very little knowledge of modding, my quick fix correct me if it's wrong.

for _,force in pairs(game.forces) do
    if force.technologies["fluid-handling"].researched then
        force.recipes["6x6-underground-tank"].enabled = true
        force.recipes["5x5-underground-tank"].enabled = true
        force.recipes["4x4-underground-tank"].enabled = true
    end
end

Sorted. Used the syntax from how it's done in the Afraid of the Dark mod and it's tested working.
New revision with this fix has been uploaded.
Thanks for your feedback and code suggestion.

hello.
I have a question.
How to get "6x6-underground-tank" on console?
I have no knowledge of this, so I don't know.
The reason for asking this is that when I install another mod, I can't see it in the crafting menu, but it can be confirmed as a locked recipe in "Helmod", etc.
I'm wondering if there's a way to activate this locked recipe, so I accidentally stumbled across this discussion.
I am not good at English, so I seek advice through a translator.
Please understand if my English is strange. :)
Thank you.

hello.
I have a question.
How to get "6x6-underground-tank" on console?
I have no knowledge of this, so I don't know.
The reason for asking this is that when I install another mod, I can't see it in the crafting menu, but it can be confirmed as a locked recipe in "Helmod", etc.
I'm wondering if there's a way to activate this locked recipe, so I accidentally stumbled across this discussion.
I am not good at English, so I seek advice through a translator.
Please understand if my English is strange. :)
Thank you.

/c game.player.insert{name="6x6-underground-tank", count=100}

In order to find this internal name of an item or recipe, you can press F4 and select the debug option "show-debug-option-in-tooltips", If you hover over an item or recipe, you can see it in the tooltip