Hi @MyLocal - thanks for using Local! I think I can help.
My assumption is something is blocking Local from writing to that hosts file. Are you on Windows or Mac? And do you have any antivirus or similar software that could be locking down that file?
Here is a related help doc (for Windows, issue is similar for Mac) around the hosts file not being accessible - Help! I'm running into an EPERM error with the hosts file on Windows. Can you try taking the steps here?