You are on the correct page to add Super Priority, so if it is only showing Standard Processing and does not give an option to select Super Priority, that means all the available Super Priority slots have sold out for the day and there are no more available.
If you definitely want to use Super Priority, DO NOT pay for the visa if the Super Priority option is not offered, because you will not be able to add it later and will have to use the standard 6-month processing instead.
Super Priority slots are released at 1am every night and sell out almost immediately (there are only about 50 released each day)... so if you want to purchase priority, you will need to log out without paying for the visa, and then try again at exactly 1am tonight.
If you cannot get one then, you will need to try again the next night, and the next, and so on, until you manage to secure one.
