Here’s some working code, broken down into simple pieces just for clarity:
>>> import datetime
>>> import time
>>> from email import utils
>>> nowdt = datetime.datetime.now()
>>> nowtuple = nowdt.timetuple()
>>> nowtimestamp = time.mktime(nowtuple)
>>> utils.formatdate(nowtimestamp)
'Tue, 10 Aug 2010 20:43:53 -0000'
Explanation: email.utils.formatdate wants a timestamp — i.e., a float with seconds (and fraction thereof) since the epoch. A datetime instance doesn’t give you a timestamp directly — but, it can give you a time-tuple with the timetuple method, and time.mktime of course can then make a timestamp from such a tuple.
EDIT: In Python 3.3 and newer you can do the same in less steps:
>>> import datetime
>>> from email import utils
>>> nowdt = datetime.datetime.now()
>>> utils.format_datetime(nowdt)
'Tue, 10 Feb 2020 10:06:53 -0000'
See format_datetime docs for details on usage.