"We lack her signature."
Translation:Vi saknar hennes underskrift.
I disagree. Although the word "signatur" is often used in Swedish as a direct translation of "signature", this is a false friend. The Swedish word signatur signifies an alias of some kind (a pseudonym, or a user name), and not the full name written by hand as a means of identification.
No, it should definitely be valid as it is often used in your everyday life in Sweden. When you are supposed to sign something, "Signatur" is oftenly found. Signatur, namnteckning, underskrift, påskrift, namnunderskrivt, påteckning should all be valid.
No, fattas is passive in its construction, so the thing that fattas is what is lacking or missing. If you say vi fattas, it means ”we are missing”.
I did Vi fattar Hennes Autograf, because that is what we use in Swedish class
fattas is a deponent verb which means that something is missing. So you could say that signaturen fattas. There's also the usage [noun] fattas [reflexive pronoun] to mean that you're missing something. In this case, it would be Hennes signatur fattas oss. But this meaning is very old-fashioned, so much that I don't think it should be accepted, lest people learn it and think it's normal modern usage.
The phrase Vi fattar hennes autograf (no capitalisation) would mean "We grasp her autograph" - both readings possible. But it cannot mean that the autograph is missing.