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Yep, that should help.
Try running composer require psr/http-message in your project directory to install it.
You can't post in HTML directly in most forums since they usually limit formatting for safety reasons, it's not this one the only one.
You can actually disable the username requirement by adjusting the settings in your config.yml file. Just look for the allow-java setting and set it to false. This should let Bedrock players register without needing a Java account.