You could use a tab or a first-line indent, but I don't know why the
spacebar wouldn't work as well. Note, however, that if you are trying to add
space at the beginning of a line in the middle of a paragraph (rather than
at the beginning), all the spaces you're inserting are piling up at the end
of the previous line (as you'll see if you display nonprinting characters).
In order to insert space the way you're trying to do it, you would need to
add a line break at the end of the previous line or enable the Compatibility
Option to "Wrap trailing spaces to the next line."