I'm trying to reinstall the same product on the same PC.

Hi @영훈 김

Root Cause :

In the Office license activation will stored the details of motherboard, OS, Hard drive(SSD). so, if you changed or replaced license will deactivate. some of the software's check the LAN port MAC address also.

Solution :

Need to reactive your license.

Step 1 — Sign in to Office with the correct account

Open Word → File → Account → Sign in Use the same Microsoft account you used during the first activation.

Step 2:

Go to :

https://account.microsoft.com/services

Look for:

Office Home & Business 2021 – Installed on: <your old device>

If you see it → the license is valid.

Step 3 :

In Word:

File → Account

Click Activate Product

Choose Troubleshoot activation

Select I changed hardware on this device

Sign in again

This rebinds the license to your new SSD installation.

If above solution not helpful do the below steps.

Step 4 — Contact Microsoft Support (they will reassign the license)

Go to:

https://support.microsoft.com/contactus

Tell them:

“I replaced my SSD and need my Office Home & Business 2021 ESD license re‑assigned to my new Windows installation.”

They will:

Verify your purchase

Remove the old device binding

Allow activation on the new SSD

Thanks.

See https://support.microsoft.com/en-us/windows/activation/activate-microsoft-perpetual-products-using-the-product-activation-portal .