"I am not going to prevent you from it."
Translation:Yo no te lo voy a impedir.
It's an odd phrasing, and I agree that no native speaker is likely to use it, but it's not wrong. It would require context to be understood. I imagine that this particular construct was used to push us into understanding the Spanish. Duo has to walk a line between presenting us translations in a way that will help us understand how they work, and presenting us phrases that people would actually say. It doesn't always get both right. A different verb choice might have helped with that here.
I think the English should be 'I'm not going to stop you' - according to spanishdict.com. 'I am not going to prevent you from it' does not make sense in English. An alternative would be 'I am not going to prevent you from doing it' or any other verb that explains what is being prevented.
'I am not going to prevent you from it'
Prevent is a transitive verb, i.e. one which takes a direct object. It is often followed by 'from' + a verbal noun (English gerund, not Spanish gerundio).
You prevent [direct object] from [verbal noun]... E.g. prevent the children from going there; prevent the car from overheating; prevent the dog from barking.
In casual speech, the verbal noun may be omitted but is always implicit.
Prevent is not used with 'from' + [something].
In English 'I am not going to prevent you from it' can only mean 'I am not going to prevent you from [verbal noun] it'. E.g. saying, doing, whatever, it.
In Spanish, this is expressed as 'I am not going to prevent that you [X] it.
X represents the verb implied but not expressed in the English. We can't know what that is in this example. Assuming the omitted verb is 'do', the translation into Spanish is:
'No voy a impedir que lo hagas'
'Yo no te lo voy a impedir'
Impedir is also a transitive verb. As in English, you prevent someone (from) doing something:
Impedir a alguien hacer algo o impedir que alguien haga algo.
I am not going to prevent you - no te voy a impedir
'Te' is a direct object
I am not going to prevent it - no lo voy a impedir
'Lo' is a direct object
Yo no te lo voy a impedir
Te is an indirect object, lo is a direct object, the translation into English is:
I am not going to prevent it for you