Unknow credentials is a status that appears in Boardgent when AMT technology of a device is active, but Boardgent does not have access to it. That could happen because Intel vPro AMT was provisioned using another method instead of using Boardgent. As an example: AMT was provisioned using System Center or manually using the computer BIOS.
How to add an AMT previously provisioned device to Boardgent
The way to connected Boardgent to AMT is executing the "Add Boardgent MPS server" action. This action will keep the AMT configuration and connect that device to the Boardgent Cloud. That action is in the device executions tab:
You must type the password used to provision AMT.
After doing that, the AMT status in Boardgent will change to Online, as shown in the picture.