You could easily cause this problem. For example, your code could contain:
newvar = put(oldvar, nlnum.);
So find out where in your program you are referring to the nlnum format.
A more complex scenario would involve this combination:
(1) You have an existing character format named $nlnum
(2) You try to print a numeric variable using this character format
But the starting point is still to examine your code and find out where NLNUM appears.
Good luck.