I found this tool for character customization but when I open it, I can’t edit the coding inside. Why is this and what can I do to fix it.
If the file you have is a .dll then you cannot modify the code. A dll file is a compiled library of code, not raw source code. If you want to view or make changes to the code, then you need to find the creator of the dll and request the original source code.
In some cases authors will be happy to do this, and only provide dll files to keep things clean and easy to work with. In other cases authors provide dll files to protect their work and they may not want to provide you source code.
A DLL is a compiled assembly and its source code is not visible to you by definition. A DLL is like an exe file which is compiled and is ready to use. In fact Unity compiles all of your scripts to DLLs at build time or when you try to run the code in editor.
Based on the license of the DLL and the library in general it might be ok to use IL disassembler tools to inspect the source of the DLL. It will not be the original source and in some parts it might be obscure but if you are doing it for learning purposes, it might be enough. However you should check the license to not do illegal things.