Dotenv not found

Well, according to the stack trace you posted it actually does.

So illuminate/support has a PHP file helpers.php that actually tries to use the class Dotenv\Environment\DotenvFactory.
When you go one level down the stacktrace you see that roots/acorn ( 2.0.6 ) broadcasting.php called the part from helpers.php:

And indeed, roots/acorn 2.0.6 has illuminate/support version range ^8.70 as dependency:
https://github.com/roots/acorn/blob/b6eeae53bcac7d321b06b9fb4316c67240e8b48b/composer.json#L52

illuminate/support on packagist.org: https://packagist.org/packages/illuminate/support#v8.70.1

illuminate/support v8.70.1 on GitHub:
https://github.com/illuminate/support/tree/v8.70.1

illuminate/support v8.70.1 helpers.php:
https://github.com/illuminate/support/blob/88939afba5b60d6bd53084ee0bd06c5e2534501f/helpers.php
And that’s strange: In the error line 646 is mentioned at where the non-existing Dotenv\Environment\DotenvFactory class is attempted to be used,
because the helpers.php of illuminate/support version v8.70.1 only has a total of 377 lines.

illuminate/support doesn’t require vlucas/phpdotenv but at least suggests that package within version range ^5.4.1:
https://github.com/illuminate/support/blob/3c1ab546c7de1f3176060b1d4d28feb100d0a37b/composer.json#L50

And the only version of illuminate/support I found that has at least 646 lines is v5.8.36:
https://github.com/illuminate/support/blob/df4af6a32908f1d89d74348624b57e3233eea247/helpers.php

So one version of illuminate/support is required (8.70.1), but another one is used (5.8.36 or similar in 5.8.x).

Very similar discussion here:
https://discourse.roots.io/t/php-fatal-error-dotenv-not-found-cannot-use-wp-cli-because-of-it-any-ideas/18844/2