move nanos check

Author: jbrockmendel

pandas/_libs/tslibs/timedeltas.pyx

@@ -2577,9 +2577,9 @@ cpdef int64_t get_unit_for_round(freq, NPY_DATETIMEUNIT creso) except? -1:
     from pandas._libs.tslibs.offsets import to_offset
 
     freq = to_offset(freq)
-    freq.nanos  # raises on non-fixed freq
     if isinstance(freq, Day):
         # In the "round" context, Day unambiguously means 24h, not calendar-day
         freq = Timedelta(days=freq.n)
-
+    else:
+        freq.nanos  # raises on non-fixed freq
     return delta_to_nanoseconds(freq, creso)