If you still couldn’t recover your Office product key, the last resort is to backup Office activation before reinstalling Windows or Office, so you can reactivate it later.
To do this, open File Explorer and browse to the location: C:\Windows\System32\spp. Make a backup copy of all its folders / files and save them to an external USB drive for safe keeping.
After installing a fresh copy of the same edition of Office, you can activate it by restoring the backup files to its original location: C:\Windows\System32\spp. When pasting the files, you may get an error messages informing me that “The action can’t be completed because the folder or a file in it is open in another program“. To get around this problem, just close the Office application and run the following command to stop the sppsvc service:
net stop sppsvc
When the files are restored, re-launch your Office application and you’ll find the product is already activated. You can also use this method to transfer Office activation over to another computer. But remember to deactivate Officeon your old PC before restoring the activation to your new PC.