What do you mean by complex?
So the issue is you are not able to set another user with a different password? correct?
I mean I am having issues to understand the query.
Hmm that would be that the user you are currently logged as does not have enough rights to make that happen,
Is there any other user with privilege level 15 that you could use?
I have 3 users on this firewall as below, please suggest me to take it further..
username mohan password zQyiXNAtatvsK5i4 encrypted privilege 15
username mohan1 password zQyiXNAtatvsK5i4 encrypted
username ishadmin password ZG1dnPYtSjPxfCBG encrypted privilege 15
Lets login as Mohan or Ishadmin and try to create a username okay.
Let me know how it goes
If you do not know them then you will need to perform a password-recovery
I mean looks like you do not know the password.