In your coupon_lines array, each item in the array needs 2 properties: code (the coupon code itself) and amount, which is either a flat fee (eg 50.00 off) or a discount (eg 10% off). Your structure ends up looking like:
"coupon_lines": [
{
"code": "MYCODE1",
"amount": "10.00"
},
{
"code": "MYCODE2",
"amount": "25.00"
}
]
If you’re already doing this – consider posting an example request here.