Add a computer to Security Group

Hi,

In Active Directory Users and Computers, I need to add a computer (Called "NPS1") which is placed under Computers as a member of the IPsec NAP Exemption Security Group.

I used the following commnad:

Add-ADGroupMember -Identity 'IPsec NAP Exemption' -Member NPS1

But I get the following error message:

Can you please, expalin to me why it is not working?

Regards,

Bader