Hi,
That's indeed possible during the checkout. But as I said, the points plugin is not made to be able to do that on an already created order.
So that's why it's not displaying as a payment method option when you pay an order on the "pay now screen".
The fact that you see it there is the mystery here. As Jerome said, it might be possible with the "virtual coupon" mode setting turned off in your payment method, but on my end, even with that setting turned off I don't see the payment method there due to other checks.
So as Jerome said, I feel like something is not adding up, like you have another user points payment method, or you don't actually have HikaShop 4.2.2 but another older version where the plugin behaved differently.
Hard to say exactly without directly looking at your backend and checking the situation with a test user account with such an order.
Could you maybe provide that via our contact form along with a link to this thread ?