Hi,


> In order answer that you have to make a MWE that we can run without the 
> need to edit a file.

> Hans

Sorry for having assumed that sharing the code was sufficient as MWE.

Here is the trimmed version that still throws the error 


\startMPpage
StartPage ;


numeric row, col, unit; 
pair  middle;
row := 1; col := 1;
unit := 2 * cm ;
picture p, q ; p := nullpicture; q := nullpicture;
string l[];
l[0] := "2" ; l[1] := " 4"; 


for i= 0 upto row :
middle := (i * unit, 5cm);
p := thelabel( l[i], middle); 
draw p;
%q := p reflectedabout (0.5[urcorner p, ulcorner p], 0.5[lrcorner p, llcorner p]);
%draw q
endfor
StopPage ;
\stopMPpage
\stoptext

The file is also attached.
 As I told earlier, an  error is thrown if I uncomment the line that assigns q and then draws it.

Thanks for your help,
ajith