Hello Anvandarin, and welcome to Eight Forums.
If the standard account is not listed while trying to change it's type using the options in the tutorial below while signed in to your administrator account, then.....
....double check to make sure that the standard account has not been disabled.
Hope this helps,
Are you able to sign in to the standard account?
If so, then while signed in to an administrator account, see if you may be able to change the standard account's type using the command prompt in Option Four of the tutorial below since you were able to see the standard account in the command prompt.