how to write Api To Upload A text file and Create a datatable

Try this

[HttpPost("upload")]
public IActionResult Upload(IFormFile file)
{
    // TODO: Implement file upload logic
    using (var streamReader = new StreamReader(file.OpenReadStream()))
    {
        var dataTable = new DataTable();
        var headers = streamReader.ReadLine()?.Split(',');
        if (headers != null)
        {
            foreach (var header in headers)
            {
                dataTable.Columns.Add(header);
            }
        }
        while (!streamReader.EndOfStream)
        {
            var values = streamReader.ReadLine()?.Split(',');
            if (values != null)
            {
                dataTable.Rows.Add(values);
            }
        }
        // TODO: Do something with the datatable
    }

    return Ok();
}