Looks like sign tx was removed from counterparty and needs to be done on bitcoin.
Signing Transactions Before Broadcasting
Note: Before v9.49.4, the counterparty server API provided an interface to Bitcoin Core’s signing functionality through the do_*, sign_tx and broadcast_tx methods, which have all since been removed.
All create_ API calls return an unsigned raw transaction serialization as a hex-encoded string (i.e. the same format that bitcoind returns with its raw transaction API calls). This raw transaction’s inputs may be validated and then must be signed (i.e. via Bitcoin Core, a 3rd party Bitcoin library like Bitcore, etc) and broadcast on the Bitcoin network. https://counterparty.io/docs/api/#signing-transactions-before-broadcasting
You can use server coinb.in to sign raw transactions.