This indicates an issue with the bluetoothd daemon. Steps are as follows. by default the daemon that handles bluetooth connections on Linux Mint is bluetoothd. Bluetoothd handles org.bluez which is the interface for bluetooth management. An outdated version of bluetoothd could cause this issue.
1.) Check the status, in terminal systemctl status bluetooth
If it is running you will see something like:
Status: Running
(I dont expect it to not be running but this will check the status)
2.) retry connection, if the issue persists attempt to reload the dbus system sudo systemctl restart dbus
3.) Reattempt, if it persists try checking your bluetoothd version
bluetoothd --version
4.) If the version is outdated ie: > 6.0 you should update the daemon.
sudo apt update
sudo apt install --only-upgrade bluez
5.) Check the version again bluetoothd --version
6.) if it is updated restart it sudo systemctl restart bluetooth
7.) this hopefully will resolve the issue so go ahead and try again