Clear it in 3 steps
Start the environment
Investigate the target
Submit the flag

In Linux, files whose names start with "." (dot) are "hidden files" and are not displayed with the regular ls command. There seems to be a hidden file on the server. Try to find it!
Use `ls -la` to show hidden files too
Look for files starting with "."
There's a file called `.secret`. Check its contents with `cat .secret`