Kleopatra is a certificate manager and a universal crypto GUI. It supports managing X.509 and OpenPGP certificates in the GpgSM keybox and retrieving certificates from LDAP servers.
Kleopatra Flatpak may not work out of box. It needs an running gpg-agent from host to work, and here are some instruction for this.
In order to run gpg-agent in the background, you can run following command from your terminal:
gpg-agent --daemon
You can add this line in your ~/.bashrc or ~/.profile, so that gpg-agent can start on your login.
Or you can utilize systemd to start it. See this for more infomation: https://sources.debian.org/src/gnupg2/2.2.39-1/doc/examples/systemd-user/README/