How to Write a printer bluetooth (hcrp) language monitor.When I use pfnReadPort, I get error code 87.pfnWritePort can be written.

Question

flag = (*pIniPort->fn.pfnReadPort)(pIniPort->hPort, pBuffer, cbBuf, pcbRead);    
if (!flag) {
    a = GetLastError();         
    MyDebug("LmReadPort:error code is %d", a);     
}