Welcome to the forum.
Indeed you can install on a second machine but without doubt
the copy you are using has been activated twice already. This can
occur if your machine has been reformatted and you did not
deactivate first. Maybe your desktop machine was changed and you
installed on the new machine without deactivating on the old one.
Could a colleague have installed it to take a look?
Go back to the activation screen and it will advise you what
to do. You need to phone the number that you can find there and
explain the problem. I had this after a hard disk failure and was
given an activation number.
Help others by clicking Correct Answer if the question is answered. Found the answer elsewhere? Share it here. "Upvote" is for useful posts.