My enchanting skill is 100

I have grand soul gems with grand items

I can't enchant anything at all with constant effect enchants, only cast when used. I click the enchant type where it says by default "cast when used" and nothing changes.

For weapons I can change it to cast when used or on strike, but no armor or clothing/rings/amulets let me make any constant effect items? I don't get it. What am I doing wrong?

Just taking a guess here, but what's the creature trapped inside the grand soul gem? If the soul size is not 400+ it won't work.

It's a Dremora Lord. I thought that was the highest soul? I wanted to do an Ogrim Titan but the soul ID is "Ogrim Titan" and as we know, you can't enter that in the console, it never works if there isn't a _ in between the names