Thank you very much for your reply. I am using Windows 10 Home, version 1803 with RStudio v1.1.456.
Trying to answer your question about if this happens with a specific worksheet or with all, I've been trying different things, and I believe I have found what the problem is. My native language is Spanish, so I have lots of folders with accents in their names (e.g. "Curso Estadística"). Whenever I try to import an Excel worksheet that is inside a folder with an accent in its name, the sheet menu shows only the Default. If I copy the exact same Excel file to another folder without an accent in it's name, the menu shows all the sheets that the file has. I've tried this with different files and different folders, and it is what occurs.
When the filie is here C:/Users/letiv/Desktop/MyData.xlsx I can select the sheet without a problem.
When the file is here too: C:/Users/letiv/Desktop/Curso EP/MyData.xlsx
When the file is here: C:/Users/letiv/Desktop/Curso EP - Estadística y R/MyData.xlsx I only see the default in the menu.
Thank you very much because you helped me find the problem with your question. Anyway, I think RStudio should enable import any Excel file, no matter where it is saves in the hard drive.