PHPmyNO over SSH, you could try going into the Flarum folder and running just composer install
Because the error says the package is not in composer.json, there must be something broken with the current Composer state. composer install should make sure the vendor folder matches with what is described in composer.json.
If this still leads to errors, you can rename your vendor folder to create a backup (vendor2 for example, just so you can rollback if needed), then run composer install. This will create a brand new vendor folder, hopefully without any problem.
If you have other extensions that no longer exist in your composer.json, the output of the command should show them in the error message.
