The nlocktime parameter in createrawtransaction seems not work using Bitcoin Core Wallet GUI If I add the CLocktime (e.g 20) at the end it always gives:

Error: Error parsing JSON:{"12CxqQHzhFxCm2TkGkKHdFFPyUerFXcSte":0.008,"1DBDDvpcHv66LD7sWz1mCitPALW1qfrga7":0.012},


If I delete the CLocktime at the end it will be ok. 
Have anyone has similar experience and know why?
You're putting the locktime in the wrong place. You have

"[{\"txid\":\"myid\",\"vout\":0}]" "{\"address\":0.01, \"address\":0.01} 20"


but it should really be

"[{\"txid\":\"myid\",\"vout\":0}]" "{\"address\":0.01, \"address\":0.01}" 20


The locktime is a separate parameter, not part of the quoted parameter that has the transaction outputs. It goes outside of those quotes.