I wanna spend my iTunes money in Family Sharing but it won’t let me

You cannot use Apple ID balance for family sharing. See this text from the Apple Knowledge base about setting up family sharing

One adult in the family—the family organizer—can set up Family Sharing for the group from their iPhone, iPad, iPod touch, or Mac. If you turn on purchase sharing, you'll be asked to confirm that you agree to pay for purchases initiated by the family members you invite and that you have a valid payment method on file.Valid payment methods for setting up Family Sharing include credit cards and debit cards.*

Only the family Organizer can use a charge card to pay for something. Read what Apple says about the very specific conditions for payment under Family Sharing:

From: How to share purchases with your family - How to share purchases with your family - Apple Support

"When you turn on purchase sharing, everyone in your family gets access to apps, music, movies, TV shows, and books that family members buy. The family organizer is billed for family members' purchases"

The second phrase is a bit over-simplified because others can still pay for their own items, they just have to use personal account balance to do so. Turning on purchase sharing simply activates the feature whereby if a family member does not have adequate personal balance to pay for something then the organizer's payment method (e.g., credit card) will be charged.

Read the document for the link to "learn how purchases are billed if a family member has Apple ID balance". This tells you how it works.

The payment method for Family Sharing is automatic:

1. If anybody in the family buys or subscribes to something, Apple first attempts to charge the item to that individual's Apple Account (Apple ID) balance, if any.

2. If a family member does not have enough personal Apple Account balance, any excess will be charged to the Family Organizer's primary payment method (usually a card of some kind). The Organizer's personal balance will not be used for purchases made by other family members. If it cannot bill the primary payment method, the Organizer will need to make another payment method the primary method.

- "Some purchases, including gifts, can't be billed to Apple ID balance and will be charged to the family organizer's payment method." "Some subscriptions might not be charged to Apple ID balance."

Ref:

- Family purchases and payments - How to share purchases with your family - Apple Support

- How apps, content, and subscriptions from Apple are billed - How apps, content, and subscriptions from Apple are billed - Apple Support

- Check your Apple ID balance - Check your Apple Account balance - Apple Support

- Change, add, or remove Apple ID payment methods - Change, add, or remove Apple ID payment methods - Apple Support

I'll add that since your family organizer status is unclear, in most situations the statement that was recommended: "You cannot use Apple ID balance for family sharing", goes completely against what Apple says in Apple's own documentation. In most cases you can use Apple ID balance. If you are not the Organizer you can only use your Apple ID balance or the Organizer's card. If you are the Organizer you can use your Apple ID balance or your card to pay for things you buy. The exceptions are if they are things that require a card for payment, and the Organizer's personal balance cannot be used to pay for things that other family members buy.

Redeem your gift card. Any family member can use only their own personal balance to pay for almost anything. The Organizer's card only comes into play in the situations I carefully outline in my post, namely if a family member does not have enough personal balance to pay for an item.

Note that the second paragraph in the recommended reply is not actually in the Apple Knowledge Base documentation that I can find with a web search. It may be from an old version of a document (no link was provided to indicate which document is supposed to say that) and you should refer to what new versions say.

Blnd