First of all, make sure that you have opened a bank bond account, otherwise you can't buy it. Secondly, your purchase is the selling price of the bank, which is generally not required to be filled in (if some bank trading systems require it, you should fill in the selling price).

Normal trading days should be no problem, but some banks sometimes can't do it at a certain time, such as after 10 pm or during debugging.

If you still have problems, call the bank's customer service, because the online banking settings of specific banks are different.

There is also a comparison of buying government bonds. Different banks offer different prices for the same government bonds at the same time.