Alright, let’s first check that you’re able to download the correct key.
Try the following to download the key to a file:
$ curl -s https://brave-browser-apt-release.s3.brave.com/brave-core.asc > brave-core.asc
and then check that it has the right size:
$ ls -l brave-core.asc
-rw-r--r-- 1 francois francois 3914 Jan 14 16:33 brave-core.asc
and that the contents are correct too:
$ sha256sum brave-core.asc
caf94b3b31d5b49dbbf26b8b05232057a4107b0b8a2a9f990774d4d1a6289a87 brave-core.asc
If you’re getting a different size or a different sha256, can you try downloading using a web browser and saving to a file. Then you can check the file size and run the sha256sum command again.