Planets are invisible to us at and near conjunctions because they are lost in the glare of the sun.
The rare exception to this is when an inferior conjunction happens and the planet is on the same plane as us and the sun. When that happens we see a spectacular transit of Venus or Mercury across the face of the sun.
Sadly, they are quite rare. To see Venus pass in front of the sun. The best time to see Venus and Mercury is when they are at greatest elongation, shown as pink rings on the diagram below. At greatest elongation, they are as far from the sun as they get from our perspective on Earth.
For Mercury that still normally means challenging viewing. It orbits so close to the sun that it is rarely visible for more than an hour before sunrise or after sunset, so we are finding the planet in the glow of dusk or dawn. Unlike the inferior planets, the outer planets never pass between Earth and the sun. At opposition, we sit directly between them and the sun, which we find easiest to think of as the sun being 'behind' us with the planet 'facing' us.