Thank you for confirming the Epic is correct one.
Please see the request I made with API companion just now. response comes back as status 200, but when I see in the list of transactions, it show the error msg. Please see the attached screenshots.
here is the body content of the request
{
"epic": "IX.D.SPTRD.MONTH1.IP",
"expiry": "DFB",
"direction": "BUY",
"size": "1",
"level": "4474",
"forceOpen": null,
"type": "LIMIT",
"currencyCode": "GBP",
"timeInForce": "GOOD_TILL_CANCELLED",
"goodTillDate": null,
"guaranteedStop": "false",
"stopLevel": null,
"stopDistance": "8",
"limitLevel": null,
"limitDistance": "7"
}