In the case that you stay using a not supported browser like Thorium, you must create a file org.keepassxc.keepassxc_browser.json in:
/.config/thorium/NativeMessagingHosts/
with this settings
{
"allowed_origins": [
"chrome-extension://pdffhmdngciaglkoonimfcmckehcpafo/",
"chrome-extension://oboonakemofpalcgghocfoadofidjkkk/"
],
"description": "KeePassXC integration with native messaging support",
"name": "org.keepassxc.keepassxc_browser",
"path": "/usr/bin/keepassxc-proxy",
"type": "stdio"
}
Save and restart your browser and try again.
Or try check in ~/.config/YOUR-BROWSER-NAME/NativeMessagingHosts/ and search for org.keepassxc.keepassxc_browser.json if the folder is empty, simply must create a file and set the data.