data sales;
input jan feb mar ;
datalines;
1 1 1
2 2 2
3 3 3
5 6 7
;
run;
proc print data=sales noobs grandtotal_label='Monthly_Sum' ;
var jan feb mar ;
sum jan feb mar ;
run;
The PRINT procedure needs a place where it can put the summary labels. For this it needs a column for which no summing is done (meaning a character column). Since your dataset contains only numeric variables (and you sum them all), the procedure can't print the label.