The term "user not found on Instagram" means something different in different situations. In order for you to understand what it means for your case, here are all possible possibilities:
1. User Not Found on Instagram but the profile picture is visible - This error occurs when you visit the user's profile when it just got disabled/suspended.
2. User Not Found on Instagram but the posts are visible - this error occur due to technical glitches of Instagram, you can fix this on its own by restarting your Instagram.
3. User Not Found on Instagram but can DM the user - Messages are not delivered and received by the user. Even if the user's account is discovered someday, those sent messages won't be reflected.
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