That's what Jacob (and also I) will be saying...and is the same error that most people make for this tutorial. After typing the command, up to the part where you type "multiboot.d/1"....remember: tap/press the "Tab" button that you see on your screen.

In Linux (particularly the Terminal), the purpose of the "Tab" button on the N900 (or the Tab key on your computer's keyboard) is for "auto-complete" of text. It kind of searches for a file that matches the initial characters that you have typed and if it finds it, it automatically completes typing it for you.

If you did not press/tap the "Tab" button, then your file will be saved as "11" only.

One other possibility is that you have tried installing NITDroid prior (with the incorrect step) and ended up saving a file that is named as "11" previously, so when you tried it the next time around - this time by pressing "Tab - you end up still with "11" because the Terminal "auto-completed" it for you and found an exact match of an already-existing file that is named "11". In such case, you may want to type the whole filename correctly, or verify if the files exists by installing and opening FileBox.