Are you attempting to modify one of the system accounts delivered as part of Solaris ? Doing so is NOT supported (beyond setting a password for the root account). Any such attempted will actually be undone on the next 'pkg upgrade' or 'pkg fix' and will cause 'pkg verify' to fail and indicate the system is broken.
If this is not for a system delivered account then you need to find the entry for the account in one of the /etc/user_attr.d/ files and remove the "RO" from the third column. Do NOT do that to a system account or one delivered via IPS package. If it is delivered from an IPS package then change the source and republish the package instead.
Oracle Solaris Engineering Security Architect