It’s possible that the search criteria you used for your estimate for Twitter tweets is too broad. Your estimate needs either a keyword, a set of keywords, or additional filters; otherwise the amount of data in the estimates for all English Tweets over a year or more will be too vast for processing. Consider refining your search criteria to limit the scope.
It's also possible that your query contains an error. Please see this document about the new Twitter Gnip PowerTrack rules.