Some heat is anticipated and perfectly normal, though the phone should not become too warm to handle comfortably.
The Galaxy A20 supports Adaptive Fast Charging, and therefore is likely to get warm during charging. This is expected, especially if the device is being operated while it is charging. If the device gets warm during normal use, please see the tips above.
Use the provided Samsung wall charger and wait at least 1-2 minutes to allow the device to charge.
If the device does not seem to charge with the provided AC charger, connect the provided USB cable to a different AC charger (if available). If the device charges under this configuration, the provided AC charger needs to be replaced.