Spell "Summon Mehrunes' Harbinger" doesn't work

Hello everyone, as the title says I can't spawn her in through the spell. I've tried a couple of things and can't load a previous safe anymore because the auto saves before were deleted for some reason.

Am I doing something wrong? I tried it in the cave where you first obtain the spell, outside, in a city. I'm at a loss...

I can't progress the associated questline which wants me to get my reward through the summon.