Ritual summon.

to ritual summon you need certain spell cards. if you press 'Related Cards' it should show you what card you need to ritual summon it

I’m having the same issue, for example, i’m trying to ritual summon crab turtle. On numerous occasions I have had 3x lvl 4 monsters on my field, the spell card turtle oath and crab turtle in my hand, yet the spell will only set and will not activate.

Read the cards

The spell ritual card can only set by me and can't be activated, please help

You must have the Ritual Spell card with the Ritual Monsters' name in the description and the Ritual monster in your hand. Activate the spell card then tribute enough monsters from your hand and/or field that total levels equal or exceed the Ritual monsters' level. Make sure not to tribute the Ritual monster. Lol

Google it -3-

Ritual Monsters can only be summoned using a ritual spell cards, the ritual spell cards are specific to the monsters that it can summon which is in the card description of the ritual spell card.

Once you have the ritual spell card and the ritual monster in your hand, you need to sacrifice other monsters that you have either in your hand or in your monster field that is the same level as the level of the ritual monster.

If you want to summon crab turtle, which is lvl 8, then you need to sacrifice something like a blue eyes white dragon or two level 4 monsters from hand or field.