For anyone else experiencing the same issue on Dietpi:
In the Security and Setup, in the Overview Settings (above the Version section), it says that Nextcloud 24 or later requires PHP 8 or higher, which I overlooked. This conflicts with Dietpi’s current PHP version of 7.3.31-1~deb10u3.
Manually upgrading to PHP 8 or later is possible but, it may cause issues with Nextcloud according to these forum posts (Post 1, Post 2). These issues may be solvable according to this other forum post, or this one but, most of these are from other Linux distributions, so the problems and the solutions may not translate.
PHP 8 will probably be available on Dietpi but, it’s unclear when, so the best thing may be to just wait (Forum post 1, post 2).