I think there’s something wrong with the way SemRush is analyzing the page. Unless something has changed, Discourse doesn’t use the Carousel schema type. There’s a post from 2022 about the issue: SemRush Structured Data - #4 by Falco
Testing https://forum.reflex.dev/about on schema.org, it’s finding ItemList, AboutPage, and SiteNavigationElement schema types. No errors or warnings are being reported. You can check the results here: https://validator.schema.org/#url=https%3A%2F%2Fforum.reflex.dev%2Fabout.