From what I see in your screenshot, you moved the top left triangle on the ruler (this symbol indicate the first line indent of the paragraph).
Each paragraph style could have a different first line indent. As long as there are selected paragraphs with different indents, the indents symbols will disappear from the horizontal ruler.
So, if do you wish to “move all the text on [the] left side” of your document, you will need to change the page margin, not the paragraph indent.
To do that,
place your mouse pointer (or equivalent) at the 0 of the ruler (or between the two “triangles”, if indents are set to 0)
- and when you see a double-point arrow, press and move to the right
Note that if Page layout is set to Mirrored, one page will have more left margin and the next more right margin.
LibreOffice Help on Using Rulers.
Check the mark () to the left of the answer that solves your question.