Is it permissible, Islamically, for an employer to deduct the cost of an unintentional error from an employee's salary, given that this condition was not stipulated beforehand and the company is large?

The private employee is not liable for what is damaged under his hand, whether it be work or equipment, except in cases of negligence or transgression, because his hand is considered a hand of trust. If what occurred on your part was not a result of carelessness, negligence, or transgression, then you are not liable. However, if it arose from such, then you are liable, and fungible items are guaranteed by their like, and non-fungible items by their value.