If I recollect, it was saying there should be a min balance of 150rs for any addition (when my account was new / with 0 balance).
If I can suggest,
- topup some amount so that its above 150
- use the web interface (and not the app) as mentioned above
- do not attempt multiple changes/subscriptions/cancellation at the same time. login, add the free regional topups and logout. Try after 2 hours for any further updates.
Also,
call CC to remove the previous topups. But make sure you add one paid topup (sports/news/movies/.... some cheap one if you dont want to spend money / dont want any paid topup) along with 99 plan to avoid problems. Plan what you want before calling CC.