Note: the second command line is optional, it is used to download source packages.
Resynchronize your package index files:
$ sudo apt update
Packages installation
Install the binary packages; the "plugins" package is highly recommended but not mandatory, scripting API packages are optional (popular languages are python and perl).
deb [arch=amd64,i386 signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/debian sid main deb-src [arch=amd64,i386 signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/debian sid main
deb [arch=amd64,i386 signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/debian trixie main deb-src [arch=amd64,i386 signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/debian trixie main
deb [arch=amd64,i386,arm64,armhf signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/debian bookworm main deb-src [arch=amd64,i386,arm64,armhf signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/debian bookworm main
deb [arch=amd64,i386,arm64,armhf signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/debian bullseye main deb-src [arch=amd64,i386,arm64,armhf signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/debian bullseye main
deb [arch=amd64,arm64 signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/ubuntu plucky main deb-src [arch=amd64,arm64 signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/ubuntu plucky main
deb [arch=amd64,arm64 signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/ubuntu oracular main deb-src [arch=amd64,arm64 signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/ubuntu oracular main
deb [arch=amd64,arm64 signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/ubuntu noble main deb-src [arch=amd64,arm64 signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/ubuntu noble main
deb [arch=amd64 signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/ubuntu jammy main deb-src [arch=amd64 signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/ubuntu jammy main
deb [arch=amd64 signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/ubuntu focal main deb-src [arch=amd64 signed-by=/etc/apt/keyrings/weechat.asc] https://weechat.org/ubuntu focal main