Issue creating Web API

found the reason, the "sn" should be "SN" match the definition in data type

Use typeof() function as shown below.

cast(
  typeof('type!{urn:com:appian:types:AWM}AWM_SN'()),
  {
    id: 1,
    sn: "XXX-XX-123"
  }
)

FYI, No need to use typeof() when referencing a CDT

Agree. The syntax is little bit tricky.

If you use () it means you are referring to empty CDT instance and you need to use typeof(). if you reference directly to the cdt type like 'type!{urn:com:appian:types:AWM}AWM_SN' then no need to use typeof().

cast(
typeof('type!{urn:com:appian:types:AWM}AWM_SN'()),
{
id: 1,
sn: "XXX-XX-123"
}
)

Thanks everyone, I have made it work.