To amplify VladaFu's comment about including the whole sentence, I first wrote "we saw her in the station" and it wasn't accepted. The next time around, I realized my mistake and answered "We saw her at that station" and it was.
The mods can only help us so much if we don't give them our whole entry.
Even if "We saw her..." is accepted, why isn't it the default? First of all, using the present perfect "We have seen..." is more complicated, without any benefit, and is less common. It's also inconsistent with the rest of the course, where present tense in Czech is sometimes translated into present perfect in English (mostly correctly, I think).
Without context, "Viděli jsme ji" doesn't specify when we saw her. When the time of a past action isn't specified, English often uses the present perfect. The time can be implied or understood from context, of course, therefore "we saw her" and even "we had seen her" are accepted.
There is no inconsistency. The Czech past tense often corresponds to the English present perfect ("Have you been to London?" can be nothing but "Byls v Londýně?" in Czech - PAST). In other situations, where the process still continues into the present, it's the Czech present tense that corresponds to the English present perfect ("I've been here for 4 years." - "Jsem tu 4 roky.") This course must show both.