How to center tables in rectangles?

You can put the text in the body of your rect, with center+horizont alignment (to center horizontally and vertically):

#rect(width: 84mm, height: 42mm, stroke: 2mm, {
  set align(horizon+center)
  text(18pt)[*Samsung 960GB*]
  linebreak()
  text(14pt)[SM863a]
})

You can replace the linebreak() with a v(...) of the desired spacing, or put the whole content in a grid (better than a table when you just want a layout without grid lines):

#rect(width: 84mm, height: 42mm, stroke: 2mm, {
  set align(horizon+center)
  grid(
    gutter: 1em,
    text(18pt)[*Samsung 960GB*],
    text(14pt)[SM863a]
  )
})

or using a stack:

#rect(width: 84mm, height: 42mm, stroke: 2mm, {
  set align(horizon+center)
  stack(
    spacing: 1em,
    text(18pt)[*Samsung 960GB*],
    text(14pt)[SM863a]
  )
})