96.1.2 argument must be a non-atomic expression

This normally means that a list, a set or something else that consists of more than one element was expected. One possible cause for this error message is a construct of the following type:

(%i1) l:[1,2,3];
(%o1)                       [1, 2, 3]

(%i2) append(l,4);
append: argument must be a non-atomic expression; found 4
 -- an error. To debug this try: debugmode(true);

The correct way to append variables or numbers to a list is to wrap them in a single-element list first:

(%i1) l:[1,2,3];
(%o1)                       [1, 2, 3]

(%i2) append(l,[4]);
(%o2)                     [1, 2, 3, 4]