corpora.Dictionary
requires a list of strings whereas you are providing only a string to the constructor.
You may want to split the string into "documents". It depends on the nature of text you have. In the worst case, when each "document" will be one string - you can split on punctuation:
import string
import re
dictionnary = corpora.Dictionary(re.split('[' + re.escape(string.punctuation) + ']', docCleaned))
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