awk on unix AS/400 (Qshell)

In linux, I can do this

echo "hello" > a.txt
awk 'sub("l", "x" )' a.txt
hexlo

But if I do it on Qshell, I get:

 echo "hello" > a.txt
 awk 'sub("l", "x" )' a.txt
 awk: 0602-562 Field $($   

I have no idea of what’s going on, cannot find much about awk on qshell online. Please help…