Mercury and Venus are closer to the Sun compared to the Earth. The best condition to observe these planets is during some time after sunset or before sunrise. In some cases, they are even visible in the day.
Many flat-Earthers think that it should be impossible to observe Mercury & Venus as they are closer to the Sun. They are wrong.
Usually, their misconception comes from the assumption that we can only see in the direction of the opposite to the Sun during the night. They are wrong. The horizon changes as our positions shift throughout the night. The condition they are assuming only happens precisely in the middle of the night, and even just near the equator.
The best time to observe Mercury or Venus is when the Sun is below the horizon. The reason is that sunlight is scattered by Earth’s atmosphere, making it glow in blue. Mercury or Venus needs to be brighter than the scattered sunlight if they are to be visible during the day.
Venus is the third brightest object in the sky after the Sun and Moon. Sometimes, it can be easily observed even at the same time the Sun is visible.
The greatest elongation of Mercury from the Sun is 28°, and thus Mercury can only be observed 112 minutes after sunset or before sunrise. For Venus, its greatest elongation is 47°, making it visible at most about 3 hours after sunset or before sunrise. If Mercury or Venus is visible after sunset, then in most cases, it will not be visible before sunrise, and vice versa.