Can you make a screenshot where you try to upgrade rank and it doesn't work? What exactly happens?

To upgrade to rank 5 you need 4 4* champs or chickens. Maybe you can show us a screenshot of the part you're having trouble with?

if your Kael is rank 4 and at his max level (40), you can use 4 champions/chickens at rank 4 to upgrade Kael to rank 5. Is your Kael at max level? 

Like I said, you'd need to sacrifice 4 4* champions or chickens to rank Kael up to 5*. It appears that you lack 1 more 4 star champ (but you can just rank up the level 30s for this). You also need to unlock the champions first before they can be used as fodder.

For visual reference: 

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PS Avoid using epics as food as much as possible lmao, I don't have food champs at the moment so I just had to show how it's supposed to look like.

Thank you