I'm trying to unlock a device and need help

I've been trying to unlock a S6 Edge that I bought used from Amazon. The previous owner used Cricket and I probably bought it right after he/she canceled their plan. When it arrived, the SIM card was still in it, it said "cricket" on the notification bar, and it was still recieving a few texts after it was factory restored. Voicemail said to pay $55 to renew Cricket. The phone had no phone number and couldn't make calls, of course.

I've sent in 4 requests to unlock the device so far.

1. The first time it said, "Congratulations! Here are your unlock instructions" and told me how to unlock it, but never provided the 8-digit code. I contacted AT&T and they said, "The device you are requesting to unlock is currently active on another customer’s account." The same day, the "cricket" on the notifaction bar disappeared and the SIM card has an pop-up saying, "Attention (SIM not provisioned MM#2)." There's also a message in voicemail, but it doesn't connect to it anymore.

2. The second request said "Unfortunately, your unlock request was denied because: The wireless phone number you entered does not match our records." Even though I've never entered a phone number when requesting to unlock it.

3. Attempt said, "Your device seems to be active on an AT&T customer’s account." 

4. I just sent in the fourth request.

The only problem I think could be causing it is that the service with cricket was just canceled and it probably takes some time to clear the service when trying to unlock it. If that's the case, does anyone know how long I'll have to wait until it's able to be unlocked? Also, I'm not sure whether I should keep the cricket SIM card in the phone or take it out.

tldr: I'm trying to unlock an AT&T phone after it was used on cricket. Requests have been denied. I guess there's a waiting period. If so, how long? Should I keep old/deactivated cricket SIM card in the phone?