I can't download any themes

I have downloaded WordPress and created my first post, but when I go to Appearance/Themes, it simply says “ERROR: The theme directory “twentyfifteen” does not exist.” I have tried uploading several of the www.remarpro.com themes, but they all fail because they “could not create directory”.

I am just starting out – this is my first blog – and I don’t want to buy a theme to start with.

Can anyone help? Thanks!