willamette-river-hg-# \out data-summary-by-form.txt willamette-river-hg-# select param, site_nbr, sampdate, min(quant), max(quant), unit from concentrations group by param, site_nbr, sampdate, unit order by param, site_nbr, sampdate, unit;
The "-#" prompt in psql means you are in line continuation mode. The \out meta-command should be issued while sitting at an initial prompt "=#".
Being in line-continuation mode means you are typing something else above it that doesn't constitute a full command - and you are still not showing us what that is. This is the reason I asked for basically a self-contained example.