As a reminder, creating a user account is always a two step process:
1. Create a person account
2. Create a user account from that person account
If the person account is inactive, you will have to reactivate it before any changes can be made to the user account.
For older user accounts that have been Inactive for along period of time (i.e. older than 5 years) the Actions button in the users' s View Details windows might not be available and therefore they cannot be activated from that window.
2. From the View Details window> Click on the "Click for Operations" button (the button with a blue drop down arrow pointing down to the right of the user's name)