Even I changed to premium, the outlook client still do not accept it.
I also cannot activate reminders.
What is the problem?
I can confirm that your account is Premium so there are two possibilities that may cause you to not be able to use Premium features:
1. You're logging into a different Todoist account in Outlook.
2. You haven't logged out and back into your account after the Premium purchase and the app or browser reads the old non-Premium cached version in it.
Please try to log out and back in, make sure you use the same account and you'll have full access to all Premium features.
This did it. :-)