Cannot delete String: it has no PK

Hi,
I have a SQLite database, each table having an autoincrement ID field as a primary key. Then I want to clear a table, I pass a table name into a routine and do...
public static async Task ClearTable(string tablename)
{

        try
        {
            lock (locker)
            {
                RWBDatabase.database.Delete(tablename);
            }
        }
        catch (Exception ex)
        {
            Debug.WriteLine(ex.ToString());
            return false;
        }
        return true;
    }

I always get a failure of Cannot delete String: it has no PK.

Thanks

Pete