Maybe try wrapping your "data" in " "'s?

I just used this on the Bitcoin Core console:
createrawtransaction '[{"txid": "TXID", "vout": 0}]' '{"Address1": Amount1, "data": "4920616d204672616374616c456e6372797074"}'
And it immediately created the rawtransaction, which includes the following when I
decoderawtransaction:
...
{
"value": 0.00000000,
"n": 1,
"scriptPubKey": {
"asm": "OP_RETURN 4920616d204672616374616c456e6372797074",
"hex": "6a134920616d204672616374616c456e6372797074",
"type": "nulldata"
}
...