first of all the Xrm.WebApi.retrieveRecord() is a javascript method, which means you are working with a javascript web resource on a form in the entity account. and from what I understand you are trying to retrieve the testColumn field from the entity account, if the field was already created in the entity account then it will always exist in the database,
my advice is to make sure you're entering the field name and not the display name, and here is a sample code to retrieve the phone and account name :
function test(){
var accountRecordGuid = null,nextLine="\n";
accountRecordGuid = Xrm.Page.data.entity.getId();
if (accountRecordGuid != null && accountRecordGuid != "") {
// Retrieve Record
Xrm.WebApi.retrieveRecord("account", accountRecordGuid, "?$select=name,telephone1").then(
function success(result) {
// Perform operations on record retrieval
var sampleValueText = "Retrieved values: ";
// Single Line of Text
if (result.name != null) {
sampleValueText += "Account Name : " + result.name + nextLine;
if (result.telephone1!= null) {
sampleValueText += "Description : " + result.telephone1+ nextLine;
}
alert(sampleValueText );
},
function (error) {
// handle error conditions
Xrm.Utility.alertDialog("Error while retrieving the Account Record : " + error.message, null);
});
}
}
hope I answered your question if not please explain more the scenario
