Can I do this?
Yes, several ways.
To make no indentation your default for all new projects, adjust the formatting how you want it in Scrivener>Preferences:Formatting. In the Main Text Style section, where you can see the sample text in the box, slide the markers on the ruler all the way to the left. Once you’ve done that and the sample text is how you’d like all your documents to appear, you can close Preferences and select all your pre-existing documents and use Documents>Convert>Formatting to Default Text Style to change their appearance. (Note that if you have any formatting you want to preserve, such a tables or lists, you will not want to do this until you’ve selected the text in sections and chosen Format>Formatting>Preserve Formatting.)
To do the same but for just that specific project, you can set up the formatting the same way but in Project>Text Preferences… and select to override the main text formatting for this project. You can then convert all your pre-existing documents in the project the same way as above.
If you just want occasional paragraphs without indentation, you can just select the paragraph and slide the ruler tab over within the document itself (cmd-R to show ruler if it’s not visible). You can also then, after you’ve removed the indent and with the text still selected, use Format>Formatting>New Preset from Selection to create a preset with that style (choose “save paragraph style”) so you can easily apply it to other chunks of your writing later via the Presets dropdown menu.