1) Uninstall any existing MS office package and then restart your pc.
2) Then copy/paste following link in browser to  download the setup file http://officecdn.microsoft.com/pr/492350f6-3a01-4f97-b9c0-c7c6ddf67d60/media/en-us/ProPlus2021Retail.img
3) Open any app like Word or excel.
4) Enter the product key and Select Activate by Telephone.

Now follow the below procedure.

1)Call on this number (make sure u dial below number with starting all zero)
0008000404454

(2) Enter the auto generated security number if asked on the telephone keypad

(3) Now Choose option carefully for which product you want to activate.

(4) Microsoft will now ask for your Installation ID, type it into your telephone keypad carefully. You can type full installation id in one go to make activation process faster.

(5) You will now be asked how many devices use this license, press 0. 

(6) The automated system will now verify your Installation ID and generate you a Confirmation ID.

(7) Type the Confirmation ID into your PC carefully and press next.