Your Python installation is not able to reach maxent or treemap.
First, check if the tagger is indeed there: Start Python from the command line.
>>> import nltk
Then you can check using
>>> dir (nltk)
Look through the list to see if maxent
and treebank
are both there.
Easier would be to type
>>> "maxent" in dir(nltk)
>>> True
>>> "treebank" in dir(nltk)
>>> True
Use nltk.download()
--> Models tab and check to see if the treemap tagger shows as installed.
You should also try downloading the tagger again.