OK, that shows the problem is with the UI or core or possibly with the add-on.
I think it’s with the UI so file an issue on the openhab-webui repo.
In the mean time, you’ll have to use an inline script for your rule action to send messages to Pushover.
You can use Blockly if you install the Pushover Block Library first. You can find it under Add-on Store → Automation → Block Libraries. Then in the Blockly editor you’ll find it under Libraries.