Add a sidebar or a second footer (or header)

Hello Support Team,

Before diving into my trouble, I’d like to say this template is just G-R-E-A-T.
I try to add a sidebar so I did some test by adding a <– template-part –> tag in my template but it’s like third templater-part is not loading :/

<!-- wp:template-part {"slug":"header"} /-->

<!-- wp:post-content {"layout":{"inherit":true}} /-->

<!-- wp:template-part {"slug":"sidebar","tagName":"aside"} /-->

<!-- wp:template-part {"slug":"footer"} /-->

The page I need help with: [log in to see the link]