Error when File Inclusion/Directory Traversal

Hello, can someone help me with the last part of File Inclusion/Dir traversal course.
Appears to the inlanefreight page for this question not accepting commands by url. I scanned, used dirsearch, burpsuite, but I ever receive a firewall advice, which block my requests.
The flag is located in /, but I also can’t find this. Have scanned a hundred.

Assess the web application and use a variety of techniques to gain remote code execution and find a flag in the / root directory of the file system. Submit the contents of the flag as your answer.