BaseURL is different for each release channel

[crash101g]

Coincidentally found/like your script while trying out Fedora Silverblue (or in my case Bazzite) and wanted Brave Browser beta or nightly build but they currently do not exist as a flatpak. Believe the issue is because the URL path is wrong/different or has changed compared to when you wrote it. Your script uses $REPO-BASEURL, but the actual URLs are different depending on if you are grabbing release/beta/nightly channel.

According to https://brave.com/linux/ (as of 12/22/23)

Release Channel:
https://brave-browser-rpm-release.s3.brave.com/brave-browser.repo
https://brave-browser-rpm-release.s3.brave.com/brave-core.asc

Beta Channel:
https://brave-browser-rpm-beta.s3.brave.com/brave-browser-beta.repo
https://brave-browser-rpm-beta.s3.brave.com/brave-core-nightly.asc

Nightly Channel:
https://brave-browser-rpm-nightly.s3.brave.com/brave-browser-nightly.repo
https://brave-browser-rpm-nightly.s3.brave.com/brave-core-nightly.asc

Notice the base URL is different for each release channel (ie release.s3.brave.com or beta.s3.brave.com or nightly.s3.brave.com)