The volume is already encrypted, so you must use Add-BitLockerKeyProtector to append a new protector. The command specifies the -ADAccountOrGroupProtector parameter with a computer account (CONTOSO\Server1$). When configuring a BitLocker volume to be unlocked by a specific service account or the computer account itself (common for non-OS data volumes that need to mount automatically without user interaction but bound to the machine identity), the -Service parameter is required. Enable-BitLockerAutoUnlock is incorrect because it does not support AD account parameters.

Microsoft Learn (PowerShell): Add-BitLockerKeyProtector

Confirming the syntax: Add-BitLockerKeyProtector [-MountPoint] [-ADAccountOrGroup] [-ADAccountOrGroupProtector] [-Service] ...

URL: https://learn.microsoft.com/en-us/powershell/module/bitlocker/add-bitlockerkeyprotector

Microsoft Learn (Operations): BitLocker Operations Guide

Discusses "Service-based" protectors for unlocking volumes on servers.

URL: https://learn.microsoft.com/en-us/windows/security/operating-system-security/data-protection/bitlocker/operations-guide