Many "free key" websites require you to download a "client" or "crack" to bypass Turnitin’s security. These are almost universally Trojan horses, keyloggers, or ransomware. You could lose your entire thesis to a hacker who demands $500 in Bitcoin to return your files.
When you submit a paper to Turnitin through a class, that paper typically gets added to Turnitin's database. If you use a class ID from an unknown source, you have no control over whose class you're joining or how your work will be used. One Indonesian educational resource warns that when using free Turnitin credentials, "it is not known what type of checking it is—whether no-repository or repository"—meaning you may unknowingly submit your work to a permanent database without understanding the consequences. Your original research could end up flagged as plagiarism in the future if someone else submits similar work.
The best way to avoid worrying about a Turnitin score is to master the mechanics of academic writing.
By following these steps, you should be able to get a free Turnitin Class ID and Enrollment Key. Good luck!