Whenever the object of a sentence is a person (or similar), you must use the 'personal a'. This is also true for groups and plurals of people, such as muchas personas. (I add "or similar" because it can also apply to pets or things like that which we personify, though it wouldn't apply to general animals).
It also doesn't apply to standins for people that don't refer to a specific person.
Finally, we don't use it with tener when tener is in the sense of "Tengo un hijo.". But it would be used when tener is used in a more physical sense, as in physically having/holding rather than relationally having.
Disclaimer: this is me as a non-native drawing from memory. Google "Spanish personal a" for a more complete, more accurate description.
Why is : I know many European friends, marked wrong? Help please.
I was also surprised to find it rejected, but in retrospect, it shouldn't surprise me. It's not very natural. Only language snobs ever really speak like that. Even so, I'm reporting it as it should be accepted.
But the next time through, when I wrote 'peoples' by accident... now that shouldn't be accepted. Guess third times a charm.