I used to be able to link the password between my iCloud account and my Mac's user account. I did so on my iMac while I was on OS X Yosemite. I've since E&I that computer to El Capitan and on my main computer (MacBook Pro) when I go to Change Password it only gives me the option to change the local password and not link it to iCloud. I'm able to link other user accounts to other iCloud accounts without an issue.
At this point, I contacted Apple Support for some more help, but they were unhelpful. After chatting then talking with 4 different people (with lots of hold time), a senior advisor insisted the only thing she could suggest was E&I or go to the Store, neither of which I want to do unless I have to (especially the E&I).
I scheduled an appointment at the Store anyway, but figured I'd keep trying some things. I decided to sign out of iCloud on my Mac and create a new account to see if it was my local account. It seemed to be fine until I logged in to the account for the first time, where it said "You can no longer use the same password for both your [accounts]" and tells me, if I try to use the same password, that they cannot be the same. (Though after signing in, I am able to change it to match my iCloud account).

What could be causing it to not allow me to link the passwords?
At this point, I contacted Apple Support for some more help, but they were unhelpful. After chatting then talking with 4 different people (with lots of hold time), a senior advisor insisted the only thing she could suggest was E&I or go to the Store, neither of which I want to do unless I have to (especially the E&I).
I scheduled an appointment at the Store anyway, but figured I'd keep trying some things. I decided to sign out of iCloud on my Mac and create a new account to see if it was my local account. It seemed to be fine until I logged in to the account for the first time, where it said "You can no longer use the same password for both your [accounts]" and tells me, if I try to use the same password, that they cannot be the same. (Though after signing in, I am able to change it to match my iCloud account).

What could be causing it to not allow me to link the passwords?