Why won’t it let me summon rainbow dragon

I am not sure

You need exactly 7. It's a weird ruling thing .

You need to make sure you have an open monster slot if you have three monsters on the field you will not be able to summon rainbow dragon because it does not tribute any monsters

I’m having this same problem. Have played Yu-Gi-Oh for long time so I understand the deck. I am using all different names crystal beats but still it won’t allow me to summon rainbow dragon. Confused as to why when I’m meeting all the criteria for the damn card.

Do they have different names, Rainbow Dragon requires all 7 of them to have different names, Also they don't specifically have to be in the GY. It says Field or GY. but Two Amber Mammoths only count as 1 Crystal Beast with different names

As everyone else said, they must be 7 crystal beasts with different names in your GY. If you have multiple copies of the same ones, but still not all seven different ones, it won't work.
If you missed that point, you're a dumbass.

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"If you look carefully here:"
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all you did was copy and paste the card's text and didn't even explain where he went wrong. You stupid dumb fuck.

Did you make sure that all the 7 crystal beasts have different names in your graveyard. If you had 2 or 3 of the same crystal beast card, along with the other different ones, it won't work. Every and each Crystal Beast card in the graveyard need to be different.

who cares, rainbow dragon is tier bronze deck

If you look carefully here: Cannot be Normal Summoned or Set. Must be Special Summoned (from your hand) by having 7 "Crystal Beast" cards with different names on your field or in your Graveyard, and cannot be Special Summoned by other ways.
This card cannot activate its effects the turn it is Special Summoned. ● During either player's turn: You can send all "Crystal Beast" monsters you control to the Graveyard; this card gains 1000 ATK for each card sent. ● You can banish all "Crystal Beast" monsters in your Graveyard; shuffle all cards on the field into the Deck.

Did you make sure that all the 7 crystal beasts have different names in your graveyard. If you had 2 or 3 of the same crystal beast card, along with the other different ones, it won't work. Every and each Crystal Beast card in the graveyard need to be different.