Raspberry Pi 4 installation error

Mono version (if Sonarr is not running on Windows): Mono JIT compiler version 5.18.1.0
OS: Raspbian (version: 10 Buster)
Description of issue:
Hi

So I’ve been searching and search on how to get Sonarr installed on my Raspberry Pi 4 (Raspbian). I’ve followed the tutorial HERE, but it just won’t work. Everything goes fine until I’m at this step:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys FDA5DFFC echo "deb https://apt.sonarr.tv/ master main" | sudo tee -a /etc/apt/sources.list.d/sonarr.list

When performing the next sudo apt-get update step, I get errors and none of the next steps work.
error:
pi@raspberrypi:~ $ sudo apt-get update Hit:1 http://archive.raspberrypi.org/debian buster InRelease Hit:2 http://archive.raspbian.org/raspbian jessie InRelease Hit:3 http://download.mono-project.com/repo/debian jessie InRelease Get:4 https://apt.sonarr.tv master InRelease [13.3 kB] Err:4 https://apt.sonarr.tv master InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EBFF6B99D9B78493 Reading package lists... Done W: GPG error: https://apt.sonarr.tv master InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EBFF6B99D9B78493 E: The repository 'https://apt.sonarr.tv master InRelease' is not signed. N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt-secure(8) manpage for repository creation and user configuration details.

I have searched and tried multiple solutions but none of them work. I’m also a newbie when it comes to Linux distributions so I have no idea if maybe I have tried to many different things that they might get in conflict with each other? If so, what directories/files/programs should I remove before trying other solutions?
Example: When the first guide didn’t work I tried to install it through they guide for BananaPi which was suggested on a fourm, now I’m wondering if the trials from guide 1 interfere when trying to follow guide 2? like broken files or something.

Thanks!

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 0xA236C58F409091A18ACA53CBEBFF6B99D9B78493
echo “deb http://apt.sonarr.tv/ master main” | sudo tee /etc/apt/sources.list.d/sonarr.list
sudo apt update
sudo apt-get -y install nzbdrone

Thank you very much!