If you get this error when trying to sign a transaction with your Ledger, make sure that the Polkadot or Kusama app is open on the device. You should see "Polkadot Ready" or "Kusama Ready" on the device.

It's also possible that another app may be hijacking the Ledger connection, usually Ledger Live. Make sure Ledger Live, or any other apps that may be using Ledger, are closed.