You must first check if below checks are enabled in security--> AAA->password policy which is restricting you to keep the usename and password of your choice..??
Configuring Password Policies (GUI)
Choose Security > AAA > Password Policies to open the Password Policies page.
Select the Password must contain characters from at least 3 different classes check box if you want your password to contain characters from at least three of the following classes: lower case letters, upper case letters, digits, and special characters.
Select the No character can be repeated more than 3 times consecutively check box if you do not want character in the new password to repeat more than three times consecutively.
Select the Password cannot be the default words like cisco, admin check box if you do not want the password to contain words such as Cisco, ocsic, admin, nimda, or any variant obtained by changing the capitalization of letters or by substituting 1, |, or! or substituting 0 for o or substituting $ for s.
Select the Password cannot contain username or reverse of username check box if you do not want the password to contain a username or the reverse letters of a username.