Aha! It's just the Excel update that won't install.
This is a server error of some sort. Either the update didn't get fully uploaded to the update server, or it's corrupt in some way. This is an issue with the server that services your downloads.
This happens to me every once in a while with apps in the beta program. My solution is to wait a couple days. The server gets updated periodically and then the install works again. The longest I think you'd ever have to wait is one week.
However, if you use the Menu Bar and choose Help > Feedback > Report a Problem and describe what's going on with the Excel update from the server and that the other apps update as expected, maybe someone on the Microsoft side can reset the server.