Solution 1:

The package libappindicator has been removed from Kali Rolling since 2021-01-19:

[2021-01-19] libappindicator 0.4.92-8 removed from kali-rolling (Kali Repository) 

To install discord use snap:

sudo apt install snapd
sudo systemctl enable --now snapd apparmor
sudo snap install discord
snap connect discord:system-observe

discord on snapcraft.

Installing snap on Kali Linux.

Edit

snapd command not found

You need to edit your sources.list through:

echo "deb http://http.kali.org/kali kali-rolling main non-free contrib" |\
sudo tee /etc/apt/sources.list
sudo apt update
sudo apt install snapd