You selected a tariff when you chose a SIM. The SIM arrives on a particular tariff and you activate it by topping up. This date becomes your anniversary date, and you can change tariff on your next anniversary date. Its all explained in the booklet with the SIM. If you bought the wrong SIM then phone O2 and ask them nicely to change the tariff for you.
You will change eventually. Give it a few days. When I first got my new O2 Sim. It kept saying I was on No Tariff but I got the text and web allowance. I asked CS and they said leave it a few days. And they were right. A few days later it said I am on Text and Web