The following steps help you to install Pass Butler for desktop on Ubuntu.
Add Pass Butler repository:
$ echo "deb https://apt.passbutler.de bullseye main" | sudo tee /etc/apt/sources.list.d/passbutler.list
$ wget -q -O - https://apt.passbutler.de/signing-key.gpg | sudo apt-key add -
Install Pass Butler from repository:
$ sudo apt update && sudo apt install passbutler
Now you should find Pass Butler in your application menu.
If you don't want to add the repository, you can download Pass Butler manually here.