I keep getting an error message in R while using twitteR
See this stackoverflow answer, it seems you can request tweets older than 7 days from the official Twitter API. See also the documentation for the searchTwitter function:

Note that there are pagination restrictions as well as other limits on
what can be searched, so it is always possible to not retrieve as many
tweets as was requested with the n argument.

Try changing the since and until dates to something within the last 7 days to see if that does work.