Reanimate from Alchemy Thunder Junction can't be added to Alchemy decks!
I bought several Alchemy Thunder Junction packs, which gave me Reanimate from the bonus sheet, but the deck builder won't let me include the card.
The card is marked as illegal in Historic and Historic Brawl. Switching the deck format to Historic makes the card appear, but if I add it and switch back to Alchemy it is marked as illegal.

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DAC169#34537 commented
That it came from a current pack is irrelevant. Bonus Sheet cards, in this case Breaking News(AKA Prosperity Post), aren't technically from whatever set of the pack they come in, so their legality goes from their last printing. For example, Clear Shot was last printed in Innistrad: Midnight Hunt making it new enough to be Standard legal but not Alchemy legal; meanwhile Back For More was last printed in Ikoria: Lair Of Behemoths, making it too old for Standard but still usable in Explorer.
For the record, there's also other legality issues; in this case, Reanimate is banned in Historic. The only formats Reanimate is legal in on Arena are Timeless and Historic Brawl.
To check set legality of cards yourself there's Gatherer at https://gatherer.wizards.com/Pages/Default.aspx and Scryfall at https://scryfall.com/
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cogdarg#73490 commented
Even though it came from an Alchemy Thunder Junction pack?
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DAC169#34537 commented
Cards from the bonus sheets can only be added to decks if it has already appeared in one of the currently legal sets for that format. Since Reanimate has not appeared in any of the Alchemy legal sets it is not legal in Alchemy.