One reason for this could be that you have not logged in to this site for more than 180 days.
However, this is required by the terms of use in order for your account to be renewed.
Otherwise it will be deleted after more than 180 days.
However, if you have newly registered to use Office 365 ProPlus, it will take at least 30 minutes for your account to be exported to Microsoft.
If you are still unable to sign in later, please check that your account name and password are correct.
You will find your account name and a note about your password after logging in under
My account.
If you cannot access "My account" and instead see "
Activate account", your account has been blocked.
Note: When entering your access data using copy+paste, make sure you do not select any unwanted spaces.
This very often leads to errors during the login process.
Therefore, please type the password manually into the field.