An empty result means the archive holds nothing matching your query. It does not mean nothing was posted.
Things to check
Spelling. Usernames and subreddit names must match exactly, though the u/ and r/ prefixes are optional.
Filters. A narrow date range is the most common cause. Try removing it.
Coverage. Very old or very recent activity may fall outside what the archive captured. Recent activity in particular lags, because the archive is not a live mirror.
Deleted accounts. Content from deleted accounts may be attributed to [deleted] rather than the original username.
What not to conclude
An empty result is the least informative outcome in archive research. It is consistent with the account never posting, and equally consistent with the archive not having captured what they posted.
Subdex will not tell you a user never posted, because no archive supports that claim.