You can change your name or surname every 30 days.
If you can't change your name or surname then you probably already have either changed your name/surname in the last 30 days or one of your profile is already verified or a profile verification request is in the process.
You can change your name and surname unlimited times (every 30 days) until you make a profile verification request. Once any of your profile become verified you can't change your name and surname anymore.
Read Profile verification section in Profile Policy for more information.