How do I transfer my Apple ID gift card to apple cash

That is not possible.

When redeeming a gift card, the amount will be credited to your Apple Account balance.

You need to contact Apple Support to request a refund, see -> Check your Apple Account balance

Check your Apple Account balance – Apple Support (UK)

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If you have an Apple Account balance that you can’t use

You can't redeem or return Apple Gift Cards, App Store & iTunes Gift Cards or unused Apple ID balance for cash, except as required by law. If your jurisdiction allows the refund of redeemed gift balances, you can contact Apple Support to request a refund. 

When you contact Apple, include the following information:

•               Your Apple ID.

•               The refund amount.

•               Your current delivery address.