When purchasing US Treasury bills via auctions with Charles Schwab, one cannot choose the order type: it has to be a market order, as shown below.
As a result, one receives the following warning:
Please note: By submitting a noncompetitive bid you agree to accept the interest rate set at auction. It is possible that rates on Treasury securities purchased at auction could be negative resulting in a loss to you. If you do not wish to proceed, click "Do Not Place Order" below. (SBS-ORDR-2098)
Why wouldn't a broker let a retail investor choose the order type when purchasing Treasury bills via auctions? I wonder if the limitation comes from the broker or the Treasury bill auction mechanism.