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Hello,
Your Windows 8.1 Pro MAK key cannot activate Windows 10 Pro after the upgrade because MAK keys are edition-specific. Once the system moves to Windows 10, it requires either a Windows 10 Pro MAK key or activation through your organization’s KMS host. The upgrade process itself is valid, but the activation state resets since the 8.1 key is not recognized for 10.
You should log into the Volume Licensing Service Center with your agreement credentials and retrieve the Windows 10 Pro MAK key assigned to your license ID. Enter that key under Settings > Update & Security > Activation > Change product key. If your agreement only covers Windows 8.1 Pro, you will need to request Windows 10 Pro licensing through your administrator or Microsoft support.
If your environment uses KMS, install the Windows 10 Pro KMS client setup key (W269N-WFGWX-YVC9B-4J6C9-T83GX) and run slmgr /ato to activate against your KMS host, provided it has a valid Windows 10 Pro KMS key configured.
In short, the MAK key for 8.1 cannot activate 10, so you must use the correct Windows 10 Pro key from VLSC or rely on KMS.
I hope you've found something useful here. If it helps you get more insight into the issue, it's appreciated to accept the answer. Should you have more questions, feel free to leave a message. Have a nice day!
HL.