Hello glchrist! Thank you for participating in the Apple Support Communities.
We understand that you're unable send money using Apple Cash through iMessages. We'd like to look into this with you to see how we can help.
The following are the requirements to send and receive money with Apple Pay:
• Have a compatible device with the latest iOS or watchOS.
• Use two-factor authentication with your Apple ID and sign in to iCloud and iMessage with the same Apple ID on any device that you want to use to send or receive money.
• If you're sending money to someone, make sure there's enough money on your Apple Cash card or an eligible debit card in Wallet.
The following resource can also help with the steps to send and receive money with Apple Pay using iMessages: Send and receive money with Apple Pay
Make it a great day!