When there is no variability for a variable within a group the SdtErr is 0. Can't be "fixed" as that comes from you data.
Example:
data example;
input x y;
datalines;
1 23
1 22
1 24
2 18
2 18
2 18
2 18
;
proc means data=example min max mean stderr lclm uclm;
by x;
var y;
run;
Note that for X=2 the STDERR is 0 because all of the values of Y are the same for that value of X.
Likely that means for one or more values of your variable TRTN your dependent variable has the same values, i.e. all of them imputed(possibly) to the same value, or before your imputation no values were missing, all the same and none imputed.