The problem likely translates because you're familiar with Excel (or are following some Excel-focussed guide).
Excel does support ranges in this way, where {1, 2, 3, 4, 5} is interpreted as parameters to the SMALL() function, but Numbers does not support this format.
The most straightforward solution is longhand: where you extract the nth values and add them:
=SMALL(D35:J35, 1)+ SMALL(D35:J35,2) + SMALL(D35:J35,3)+ SMALL(D35:J35,4)+ SMALL(D35:J35,5)
There was a similar thread recently about other approaches, but I can't find it off-hand.