Keep the side navigation if the form is part of the rest of the website (i.e. the user found the form by clicking somewhere in the nav, or on a link on another page). Navigation is important to tell users where they are and where they can go. Without it, they become disoriented and confused. Moving the user around a series of pages with and without navigation breaks the Consistency and Standards UX heuristic.
Get rid of the side navigation if this form should be on a landing page (i.e., the user found it in via an external inbound link like an ad or search engine, and the only function is to get the user to fill out the form.) In that case, you don't want the user exploring the rest of your website before filling out your form. If this is your scenario, you'll need to add enough additional information to the page to make it clear what the user will gain from entering their information.
Looking at your mockup, it looks like the user is trying to add content, which is a function of your web platform, so keeping the side nav would be correct.