Translation:We are sitting at the thirtieth table.
All through the Adessive Case (lesson 3) both "nál" and nél" were translated without failure as "by" here it's not accepted, only "at"
Here I'd thought they'd corrected it. We generally sit at tables rather than by them, but stand by them more often that at them.
by the table is a perfectly natural translation to this one. it is a bit frustrating that sometimes only "by" is accepted and sometimes "at". so i'm forced to think what was the logic used on here before instead of thinking of natural translation. please fix this