Correct grammar。

app/admin_service.py

@@ -136,10 +136,10 @@ def user_expiry_time():
 
     username = request.args.get("username", "")
     if not username:
-        return "Username can't empty"
+        return "Username can't be empty."
 
     user = get_user_by_username(username)
     if not user:
-        return "User not exist"
+        return "User does not exist."
 
     return user.expiry_date