This error is expected and by design because these types of AccountShare records are automatically created and maintained by the system when a Territory is assigned to the Account.
The correct approach to remove an AccountShare established via Enterprise Territory Management assignment is to remove the Territory that's assigned to the Account which in turn, automatically removes or deletes the corresponding AccountShare.
An Object Territory Association Cannot Be Created Or Deleted With Territory Assignment Rule Association Cause.
Status: INVALID_OPERATION
000380273

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