How do I unlock all the armor?

[Bastila Shan]

Some sets only have one or two pieces that I can craft with the rest being “?” How do I unlock them?

[Катюша]

Some sets only have a few pieces, and not complete sets.

[Killance]

Some armor only has one or two pieces. Been like that for years.

[Bastila Shan]

Oh ok it’s confusing.

[Death by Glamøur]

You are probably on very early stages/chapters (Low Rank); I'm sure you're asking because of Rey Dau or a similar set.
However, if you only see a single set piece, like the Gajau or Guardian Seikret sets, that's the only piece :)