A little help with a spell, please?

I'm trying to make a celestial warlock with only the free Roll20 stuff, so I'm custom adding everything. But For some reason, my eldritch blast is not registering. It appears on my spells list, but that's it. It doesn't show up on my attacks list, and when I try and choose an invocation for it, it says it needs the spell as a prerequisite, even though I have it. Is there any way I can get this to work?

Go to your spell list, hit the (i) to open the description of Eldritch Blast, then hit the Cog symbol to edit it. Change the Output from Spellcard to Attack, then make sure it has the right attack and damage information. I never use Charactermancer, so I don't know if it being on your attack list will let you choose the Invocation, but you can always add it manually, of course.