Hi gang,

First things first, I hope everyone is doing well and keeping safe. This is going to be a fairly lengthy post, but I think it will help. 

Andrew: Is there only one version of office installed on your machine.  Sometimes this problem occurs if there are two concurrent installations running with one being active and the other though inactive is still installed. If this is the case, try uninstalling both, then re-installing your default version. 

If that doesn't solve things you may have to Change the permissions on the registry key. I would suggest that if you aren't comfortable with registry editing, make sure that you have assistance with running this step in case something goes wrong.

1. Open registry by typing regedit in run.
2. Search for the office productguid in HKEY_CLASSES_ROOT\Installer\Components\
3. Right Click the Sub Key that starts with the following syntax:

'F1291BD604B8604......'
4. Select Permissions, Add 'Everyone' and give read only Access to that group.

I hope this helps. 

Tony