We've had a couple cases where the user has downloaded and installed Office from the portal using the SMU account but then the activation window doesn't accept their credentials. This can happen if they have or had another install of Office but have either since uninstalled or it is still present. I'm not certain that this is always the case. Here is the fix that worked for an individual. Please update any feedback if this does not work but another solution has.
https://support.office.com/en-ie/article/error-0xd000000c-when-activating-office-for-mac-da865931-4658-4829-ba2d-8133390c6d25