Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Dear Kira Kriukova
Thanks for choosing Microsoft Community.
Based on your description, I know the scenario you met. For this error message, we recommend you try to clear the browser caches and re-sign in to the Office 365 admin center with the browser's InPrivate mode and then try again.
If you still meet this, please try to change a browser and check the result.
In addition, if you are trying to add a trial subscription to your current subscription, I suggest we can take a look at the steps in following article to see if it helps: Try or buy a Microsoft 365 subscription.
If there is any update, please feel free to let me know. I will continue to help you based on the information you provide. Your undersatanding and co-operation are hgihly appreciated.
Sincerely,
Katherine