If you are seeing Activation Error code 0xC004C001 on your Windows 10 computer with the message as "The activation server determined the specified product key is invalid", then its problem with the type of key.

There are two types of Keys Microsoft offers for the enterprise. One is MAK key and the second is the KMS key. In this case, you need to check if the this is a MAK key provided by Microsoft. To get this verified, you will need to contact the Contact the Microsoft Activation Call Center to verify that the MAK is valid.

It is possible that limit on the number of activation has already reached, and you may need a new key.