Step 1:

When you begin activation, Windows will try to use your internet connection to activate online. If you have problems activating online or don't have an internet connection, you can activate by phone.

To activate Windows 8.1 using an internet connection:

1. Select the Start button , type PC settings, and then select PC settings from the list of results.

2. Choose Activate Windows.

3. Enter your Windows 8.1 product key, select Next, and follow the instructions.

Step 2:

To activate Windows 8.1 by phone:

1. Select the Start button , enter PC settings, and then select PC settings from the list of results.

2. Select Activate Windows, and then select Contact customer support for help.   You may be asked for an admin password or to confirm your choice.

3. Select your location (or the location nearest to you) from the drop-down list, and then select Next.

4. Call one of the available phone numbers listed. An automated system will guide you through the activation process.

If you're experiencing activation issues, see Why can't I activate Windows and Get help with Windows activation errors.

Step 3: Contact Official Microsoft Support on Live Chat

Step 4: Still Receiving error while activating windows key after trying 3 steps ?