They also have an email address so that you can make a GDPR request via email since the EU legislation gives you the right to do that, you shouldn't be forced to use their platform to do that as otherwise it would be too easy for a company to lock you out of the process.
Here's what Reddit's privacy policy says:
If you have questions or are not able to submit a request to exercise your rights using the mechanisms above, you may also email us at to [email protected] from the email address that you have verified with your Reddit account, or submit them here.