If your BT mobile plan comes with 6GB data or more you can order the free BT Sport mobile app at www.bt.com/sportonthego
Once the order completes you will be able to login and watch it on your Android or ios device.
When did you make the changes and did you change anything else on your package ? I would guess there is an open order on your account that hasn't closed yet, most orders close the following working day, so if you did it Friday the order should close on Monday, which would allow you to order the sport on Tuesday.
If on Tuesday you still get that error, post back here and we can see about having a moderator take a look, only the mods are BT employees.
It's free as long as your BT mobile has 6GB data or more, it remains free when you're out of contract as well.
When it says you need to activate your account,you have set up a BT I’d haven’t you?You need this to log in to the app.
The OP has a BT ID, you can't order it without one, we've ruled that out already.
On Tuesday it can be ordered, that error means there is an open order, which has been confirmed as the changes were made yesterday, right now there is nothing to activate and It won't need activated, it'll work once ordered and that order completes.