>>> mxd = arcpy.mapping.MapDocument("CURRENT")
>>> df = arcpy.mapping.ListDataFrames(mxd, "City of Plains View")
I am having problem running the above lines of code on ArcMap 10.5.1. Can you please let me where I have been having problem to run it.
Is your data frame called exactly "City of Plains View"?
Yes it is called City of Plains View.
So to ask the obvious... was arcmap open with that project open?
Also check for a trailing or preceding space in the dataframe name if capitalization is correct