Column 'xxx' does not exist. It may have been deleted by another user. /Lists/Test C# what could be the reason ?

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.NET1.0

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Am trying to add items to share point list using C # code.
But am getting error
" Column 'xxx' does not exist. It may have been deleted by another user. /Lists/Test "
I dont know why am getting this error, all the fields whci am trying to call from program is available in List. but still am getting error.

Below is the code

ClientContext clientContext = new ClientContext(SiteURL);
               SP.List oList = clientContext.Web.Lists.GetByTitle(ListName);
               ListItemCreationInformation itemCreateInfo = new ListItemCreationInformation();
               ListItem oListItem = oList.AddItem(itemCreateInfo);
oListItem["xxx"] = ExcelData.Tables[0].Rows[i]["xxxx"].ToString();
               oListItem.Update();
               clientContext.Credentials = new System.Net.NetworkCredential(UserName, PassWord, Domain);
                  clientContext.ExecuteQuery()

internal name of my field contains %%
Any solutions?
Please help