The method used seems to have been patched, you can try it on your own if you want or you can update the code to make it work.
Working TikTok Username Checker to check a list of usernames for valid and invalid ones, the program sorts usernames into different txt files based of if the usernames are available or unavailable, the program currently has bugs but somewhat works, it is a proof of concept to show what can be done with some simple python programming and I haven't and will not run it on a production version of tiktok as it IS agains't their Terms Of Service.
Download the project files then follow the instructions below.
Open usernames.txt and add some usernames following the bellow format:
username_1
username_2
username_3
username_4
username_5
You can run the checker by doing the following command:
python checker.py
OR
python3 checker.py
A folder with the name data should then be created, inside the folder will be 2 files, available.txt and unavailable.txt, usernames that can be used will be added to available, and non usable usernames will be added to unavailable.txt, you can then use the username list how you wish.
By downloading and using the Program you accept FULL responsibility, It is not my fault if you decide to use it on the production version of TikTok, by doing that you are breaking there Terms Of Service which is not my fault, I have not and will not use this on a production version of TikTok and have only used it on a TikTok website clone I made and using my PC.