Fix bug in warning

Author: nsarang

pandas/core/reshape/merge.py

@@ -673,8 +673,8 @@ def __init__(
         if _left.columns.nlevels != _right.columns.nlevels:
             msg = (
                 "merging between different levels is deprecated and will be removed "
-                f"in a future version. ({left.columns.nlevels} levels on the left, "
-                f"{right.columns.nlevels} on the right)"
+                f"in a future version. ({_left.columns.nlevels} levels on the left, "
+                f"{_right.columns.nlevels} on the right)"
             )
             # stacklevel chosen to be correct when this is reached via pd.merge
             # (and not DataFrame.join)