What exactly can Excel do that LibreOffice Calc cannot do?

Microsoft Excel and LibreOffice Calc are comparable. Calc may not be as user-friendly as Excel, but in a lot of ways, it is notably more capable as it's oriented towards a complete set of features over ease of use. In fact, the compatibility between the two is rather high, but with a slightly different user interface. 

These days Excel on-line creates files in the Open Document Format, should you not have installed a compatible version of Excel on your computer. There are, of course, differences for advanced users, but in terms of the apps doing the same thing differently, they are similar. LibreOffice cannot do MS Visual Basic but has its own version of Basic

For the most part, Calc and Excel work the same way. If you're a light spreadsheet user, you probably won't notice much difference between the two. Both can do basic calculations, handle light math, and organize tables in a variety of ways. However, if you use a lot of Excel macros and programming, then you're going to struggle with Calc.

The reason is that Calc has its own macro language and it's not always compatible with Excel's VBA format. This means if you're trying to switch over to Calc from Excel, you may need to redo a lot of your macros. That said, Calc macros do translate well to Excel—just not the other way around—so if you're just sending off your own spreadsheets to other people, Calc may suffice.
The Bottom Line

If you're new to spreadsheets or just a light user, Calc gets the job done. However, if you're working in an environment where you're sharing a lot of macros then you're best off sticking with Excel. Both of them are easy to get started with, support the same file formats, and offer a pretty close selection of features. Let me know if you have more questions.