One can avoid this error by putting a dummy analysis variable on the end of the COLUMN statement. For example:
proc report data=test headline headskip split='~' nowd spanrows;
column n vargrp trt, (timer, (statc)) dummy;
define n /group noprint;
define vargrp /group order=internal style(column)={cellwidth=1.2in} left ' ' id;
define trt /across order=internal style(column)={cellwidth=0.6in} center ' ' ;
define timer /across order=internal style(column)={cellwidth=0.6in} center ' ' ;
define statc /display style(header column)={cellwidth=0.6in} center ' ';
define dummy /computed noprint;
run;