[SalesForce] lightning:recordEditForm – ERROR: Required fields are missing: [Account__c]

You would typically create a Quick Action that calls a controller method, which in turn fires off a force:createRecord event. The example in the documentation provides the necessary code:

var createAcountContactEvent = $A.get("e.force:createRecord");
createAcountContactEvent.setParams({
    "entityApiName": "Contact",
    "defaultFieldValues": {
        'Phone' : '415-240-6590',
        'AccountId' : '001xxxxxxxxxxxxxxx'
    }
});
createAcountContactEvent.fire();

All you need to do is create a new component that serves as a Quick Action to initialize your component, then fire off this event. The platform will take care of the rest for you.

Note that you can now use the standard API names, instead of maintaining custom settings that keep track of the field ID. You may need to combine this with force:recordData in order to bring in current record information.

There's other (recent) examples of this if you just search for the term here on the Salesforce StackExchange.

Sorry if my previous explanation wasn't clear. If we are going to set the value of master picklist using controllerJS, then it is recommended to rerender the dependant picklist for changing the dependant values.

i did this and it works !!

i set the value of refreshFlag to False before setting the master picklist field value and once it is done again i changed the boolean refreshFlag to True. THis rerender the dependant picklist component and the correct values gets reflected.

<!-- dependeant picklist goes here -->
<aura:if isTrue="{!v.refreshFag}">
  <lightning:inputField fieldname="dependantpicklist__c"/> 
</aura:if>