Yes, Super Priority is only offered on the visa payment page. However, due to Covid restrictions, they are currently only offering a limited number of Super Priority slots per day (I think 50).
So, if it is not offered when you go to submit the application, it’s because all the slots have already been taken for the day and there are none left for you to purchase.
New slots are released at midnight each night, so your best bet is to try again at midnight and see if you can get one.
If not, you can keep trying on different days until it will let you purchase it.
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