No, it shouldn't be sealed off completely. Think about it logically in terms of gas exchange. Plants give off O2 and take up CO2. What happens when the CO2 in a sealed container is depleted? The plants die. We don't want that. The container should have ventilation to allow for gas exchange.

plants also produce carbon dioxide and use oxygen for respiration really noticeable at night when photosynthesis isn't happening. But you do need to leave some ventilation mainly to control humidity levels in the container. This will reduce the chance of fungal/mould development.
make sure the light levels are suitable for the plants you are growing as with out light they will die.

At night, plants use up more oxygen then they produce so they need ventilation for proper gas exchange which is essential for cellular respiration.

Thank you all, I was looking at terrariums in a garden centre recently and they all had cork stoppers. I have a lovely glass container I wanted to use but without a top, so your answers have helped me. Regards.

years ago [70's] there was a fashion for using carbouys and they had a cork stopper so you could add water or allow ventilation to reduce condensation. I wish I had kept mine but I was young and wanted the space for something else. regret it now, older and wiser!