I have disabled ALL extensions (3rd party and 1st party) excluding "English". When I run the upgrade command it says "Nothing to install or update".

[~/flarum]# composer update --prefer-dist --no-dev -a --with-all-dependencies
Loading composer repositories with package information
Updating dependencies
Nothing to install or update
Package mtdowling/cron-expression is abandoned, you should avoid using it. Use dragonmantank/cron-expression instead.
Package oyejorge/less.php is abandoned, you should avoid using it. No replacement was suggested.
Package zendframework/zend-diactoros is abandoned, you should avoid using it. Use laminas/laminas-diactoros instead.
Package zendframework/zend-escaper is abandoned, you should avoid using it. Use laminas/laminas-escaper instead.
Package zendframework/zend-httphandlerrunner is abandoned, you should avoid using it. Use laminas/laminas-httphandlerrunner instead.
Package zendframework/zend-stratigility is abandoned, you should avoid using it. Use laminas/laminas-stratigility instead.
Generating optimized autoload files
Carbon 1 is deprecated, see how to migrate to Carbon 2.
https://carbon.nesbot.com/docs/#api-carbon-2
    You can run './vendor/bin/upgrade-carbon' to get help in updating carbon and other frameworks and libraries that depend on it.

ScottBeeson you need to uninstall everything listed (except those from flarum) from the command:

composer why-not flarum/core v0.1.0-beta.12

Use:

composer remove <extension>

Replace <extension> with eg fof/upload.

      Oof. The upgrade instructions in the announcement clearly just say "Disable 3rd party extensions". I'll uninstall them...

        ScottBeeson You only need to uninstall the ones listed in the first command luceos provided. Your other option is to wait for those extensions to update.

            It would be nice if that was reflected in the upgrade instructions.

            Looks like it's dem13n/nickname-changer causing the issue.

            I removed it and the upgrade succeeded. Thanks.