darse de alta
I don't understand the question.
Notice that in our dictionary darse de alta is to enroll a member. dar de alta a alguien is also used for enrolling a member.
When I threw you sentences in the machine translator to check them it used to enroll for the pronominal usage in your first sentence and discharge for your non-pronominal usage in the second sentence.
It says the first sentence is similar to:
The user hasn't been able to be enrolled.
The second sentence was similar to:
The account has already been activated or registered.
Better wait for a native.
Dar de alta has at least three different meanings. The first meaning is to sign-up or register for a service. For example, you would dar de alta with the phone company, an insurance company or a health club.
Dar de alta and dar el alta mean to be released or discharged from a hospital. (El médico le dio el alta al paciente. The doctor discharged the patient from the hospital.)
In the context of computers and business, dar de alta may refer to activating a client’s account or uploading/adding information to a computer database.