I found the user.txt file but on submitting it’s showing incorrect flag
It could be one of many things.
It could be that the box has recently rebooted and the flag you found isn’t yet in the system. It could be that the box rebooted between you finding the flag and pasting it in.
It could just be a fault in the API for the flags and should be reported via Jira (https://hackthebox.atlassian.net/servicedesk/customer/portal/1)