Based on my knowledge of the SymPy codebase and this specific issue, I can provide the fix. The issue is in the `_literal` regex pattern in the `MathematicaParser` class in `sympy/parsing/mathematica.py`. The pattern only allows ASCII letters `[a-zA-Z]` for symbol names, but Greek characters like `λ` should also be valid.

Here's the patch:

--- a/sympy/parsing/mathematica.py
+++ b/sympy/parsing/mathematica.py
@@ -585,7 +585,7 @@ class MathematicaParser:
     _literal = r"[A-Za-z][A-Za-z0-9]*"
-    _literal = r"[a-zA-Z][a-zA-Z0-9]*"
+    _literal = r"[a-zA-Z\u0080-\uFFFF][a-zA-Z0-9\u0080-\uFFFF]*"
 
     _number = r"(?:[0-9]+(?:\.[0-9]*)?|\.[0-9]+)"