GeoGen Script reference
GGE1307

A read only expression was used as a left-expression with a assignment operator such as =, += or ++.

Only variable references and array item references are legal as left-expression.

var a = 5;
var b = 3;
var array = {1: 11, 2: 22};
a += 5; // OK
Sin(a) += 5; // GGE1307, function call is not a legal left-expression
array[5]++; // OK
a + b = 3; // GGE1307, sum is not a legal left-expression