I am trying to use a singularity in a craft. I saw the website saying that "item:endless:singularity.withTag({type: "diamond"})" should work (I just changed the "diamond" to the type of singularity that i want) but when I join the game it recognize as a singularity with no tag and idk what I am doing wrong. If you can say what I am doing wrong and the right way to do it, thank you!

I found the same problem,XD

The problem is that the singularity doesn't exist, because none of what the mod adds is dynamic, it's all hard-coded !
The only singularities to exist are those in the Creative Tab, which have an NBT text (like diamonds) to use them in scripts or with the /give command and stuff.

If you want another material of singularity, one that is not directly in the mod, either use ContentTweaker to make it, or find a mod that adds it already (I remember ExtendedCrafting offers a very easy way to create singularities from any material from every mod, with customized colors, but only one color and not changing)