can’t ampere
To start charging the panel voltage has to be 5V higher than the battery. If you have a lithium battery that is fully charged then 18.4V may not be high enough.
Another cause is poor connection between the charge controller and battery. Use a multimeter to check the battery voltage at the terminals of the battery and the charge controller output terminals and make sure they are the same or very similar.
Tell us more about your controller, battery and show a screenshot of the VictronConnect app if you have Bluetooth.