Can't use JSON-RPC

[xueqiubot]

start with:
nohup exchaind start --chain-id exchain-66 --mempool.sort_tx_by_gp --home ${EXCHAIND_PATH} --p2p.seeds $EXCHAIN_SEEDS & > nohup.out

I try:
curl -X POST --data '{"jsonrpc":"2.0","method":"web3_clientVersion","params":[],"id":67}' -H "Content-Type: application/json" http://localhost:26657

but get error:
{
"jsonrpc": "2.0",
"id": 67,
"error": {
"code": -32601,
"message": "Method not found"
}

How to fix it ?
Should i wait syncing end ?