I’m unable to purchase anything with the money on my iTunes

You may need to Add a payment method to your Apple ID – Apple Support (UK)

• If you want to change your card, add first the new card, then remove the old one.
• or if you use your Apple Account balance, you need to set your Apple Account balance as your default payment.

See also ⤵️

Check your Apple Account balance -> Check your Apple Account balance – Apple Support (UK)

Add money to your Apple Account balance -> Add money to your Apple Account balance - Apple Support

If you have set your Apple Account balance as your default payment method, your default payment method will be used first by Apple when you need to make a payment.

If you update your payment method or add funds to your Apple Account balance, It would take about 1 to 3 business days in order to process.

If you're still unable to update your payment method, Contact Apple Support.

Get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK)