Is there an alternative I can use to sign a message since coinbase doesn't let you?
All previous transactions were on coinbase..
You can not sign a message on an exchange or custodial wallet. Also you can not sign a message with an address that an exchange or custodial will generate you.
Are you talking about the Coinbase noncustodial wallet? Import the seed phrase on Electrum, Sparrow or Bluewallet wallet and sign the message from any of the wallet.