"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.
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.