If you are trying to change master password in Passwords settings then there is a bug and you need to use new master password which is at least 4 characters long (but the error message doesn't mention that), we are working on settings page redesign and we will fix this issue with that.
For synchronization you need to setup master password anyway as we need something we can use to protect a backup we will send you once synchronization is set up to mail box. If you have synchronization group already set up, then you can use backup link from your email to join device without master password to your sync group.
Note that once master password is set it can't be removed.